While doing some more poking about when the car was still partly disassembled and found that the exhaust blow is probably not from the downpipe-centre exhaust join - where I thought it was - but from the manifold, based on this:

That looks to me like the flange that butts up against the engine block has rusted away underneath #3 exhaust header and its blowing down the front of the block. Bother...

On the basis that the flat mating surface to the 'east' of that bolt looks to have rusted right through, I came to the conclusion I needed a new Mani-cat which was not ideal, but it wasn't going to fix itself now was it?

The car had no radiator fitted so access was good for sorting the exhaust right there and then. My fear was that if I left it, the exhaust gasses could eat into the cylinderhead casting which would be very bad, so I really needed to get this going sooner rather than later.

What I was most concerned about was how likely is it that I will snap one/several/all the bolts trying to remove this manifold? Based on other bolts on the car which haven't been hot/cold cycled for 17 years and counting, the snap ratio was about 2:1 in favour of 'not snapping' but most of the ones that have sheared off have been relatively easy to drill out and replace with new m6 bolts. Snapping one of these will be suboptimal.

I spent a week soaking all the bolts in penetrating oil, then one evening decided to take the proverbial bull by the horns and see if I could crack off on the the more accessible bolts holding the manifold to the cylinderhead. I tentatively applied some torque from a straight bar with a hex-socket on the end and the bolt clicked, then began turning. In an astonishing turn of events, all of the exhaust manifold bolts came off without snapping and having removed the front bumper and radiator support bar I was able to ease the old manifold out. It was definitely toast, the mating face had just crumbled below exhaust ports for cylinders 3 & 4

So this was destined for the bin, possibly being sold for the metals inside - though it seems to lack any form of part number.

I had placed an order for new bolts for both the manifold > cylinderhead and manifold > centre section of exhaust from Toyota, I could recycle the old ones but for a modest price I got a set with everything I needed in it. With the manifold and gasket removed, I cleaned up the mating face of the cylinderhead

After taking this pic I cleaned it up more with a wire brush, stanley blade and a fine sanding block, I wasn't sure if it had to be cut back to bright metal or not, but decided that I didn't want to be too aggressive on soft aluminium so made it as flat as I could with gentle pressure from the sanding block.

I needed to do two things, firstly remove the rather stuck lambda sensor from the replacement manifold, this was left soaking in some plusgas to free it off; then once its free I could take the manifold to an engineering shop to skim the mating face which is a bit untidy but nowhere near as bad as the old one. I also needed to vacuum off the front of the block while its all apart so it was as clean as it can be.

The last parts I needed arrived a few days later so I set about reassembling everything. With the front bumper and radiator support bar removed it was quite an easy job. I had cleaned up the mating face of the cylinderhead with an aluminium oxide sanding block so it was smooth, located the new gasket on the two studs on the bottom edges, below cylinders 1 & 4 and then turned my attention to the manifold.

I’d taken it to my local engineering shop and they had cleaned up the mating face on their belt sander, I asked about having it skimmed, but they were concerned that the material wasn’t very thick, and also that it would be hard to clamp into position without making up a jig so I tidied it up and this was what it looked like

There were some dips around the pipes, but I hoped that the gasket would sort them out. Next I tidied up the threads of the two captive nuts with a tap and some light oil, one was Ok but the other was a bit crusty.

I wound the new spring bolts into place to make sure they fitted correctly. All good here. Next I fitted the new conical crush gasket which was a right old carry-on. I had to nip around the edges to get it to fit over the tail end of the manifold, then tap it into position with a small hammer. It was quite a lot bigger than the old (crushed) crush gasket, which would come back to cause problems later.

With this done, I could wiggle the new manifold into position and loosely fit the nuts and bolts holding it to the cylinder head. I used new nuts & bolts from Toyota for this to be on the safe side.

I refitted the lambda sensor connector then started to fight with fitting the centre section of exhaust to the manifold. As the crush gasket was uncrushed, this was a right old war, the gasket was too big, so I couldn’t get the threads of the spring bolts to engage with the threads of the captive nuts in the manifold. I eventually managed to use a small g-clamp to pull the flanges together enough to get the threads to engage and tightened them up, though I wasn't happy with the right hand side one and needed remove and refit it a few days later with a washer to make it pull in straighter. Maybe it’ll crush down better when its been heated up a bit?

I started the car up to see if it leaked and was pleased to hear it less noisy with no visible or smell-able combustion gasses at the engine end. I couldn’t let it run very long as I was in the garage with the doors closed but initial impressions are good.