Been a while since I posted and you chaps have all been so helpful, so an update (not hugely exciting but hey!). Long post apologies!
I talked to Geoff at wins international who was very helpful and happened to have a good condition large crank 1300 from an aborted restoration. I snapped it up and got a pallet to my house.
On looking into it, it seems that the engine had recently been rebuilt and possibly not even run in. Internals either new or virtually new, it was absolutely spotless.
I spent a Saturday taking the old engine out (with the help of a friend) and then another Saturday swearing a lot while trying to mate the new engine to the gearbox on my own, using my engine crane on my small gravel drive. I think I did a dance when it finally clunked in after hours of frustration.
Plumbed everything back in and it fired up, miracle. There was an annoying ticking which I was pretty sure was a leak at the manifold, but I could still drive it.
I've been busy since then, refitted the manifold and carbs and now no leaks.
Also fitted a spitfire accelerator instead of the flat herald type, way nicer on my foot (in my mind anyway).
Fitted 2 speakers behind the door cards, there is a perfect speaker sized hole. Now it has 4 speakers up front (2 in footwell door pods I made from wood and covered, nice and hidden, 2 in the doors) a new Sony he'd unit that I've mounted in reach but under the dash, plus a sub woofer in the boot (ridiculous I know but I love it).
Replaced a fuel sender, after some initial irritation that I bought the wrong type (I got a screw one type rather than the one with bolts), so now I have a working fuel gauge. And a garage that smells of the fuel due to various accidents...
Also managed to avoid what could have been a catastrophic failure due to the rocker cover. I had an aftermarket alloy one fitted, the gaskets kept breaking up so I bought a thinner wide one designed for an alloy cover. On starting the engine, I could hear a loud banging, and the rocker had worked itself loose. See the small circles/dents where they were hitting? The tops of the rockers were hitting it, partly due to less space in the rocker vs standard, and due to the thinner aftermarket gasket.
If I'd left it much longer I'm sure small bits of metal would be flying around the head. Switched back and all ok now.
Fitted the interior back in, not a perfect fit but looks fine in black. Very much helps with comfort and road noise.
Last job was the leaky bodged exhaust I built a while ago. Not only had it been damaged by speedbumps, but it was also leaking around a few joins. Bought the right bits of pipe to build a side exit exhaust, partially to help with clearance but also just because I've always wanted a car with one.
Made the error of buying stainless, thinking a local garage could tig it together. No one could. So I put it together with some decent wide clamps and plenty of putty, it's as solid as I could hope for. All fitted up, at 4 different points underneath. Now no leaks! It's loud, ridiculously loud, I'll fail any noise checks, but we'll deal with that closer to when I hit a track...
You can see where it comes out, just in front of the driver side rear wheel. I was able to attach it to chassis points all the way, so it's nice and solid (used bushes/hangers).
I had some rubbing on the front tyres from the arches and it being so so low (which I like). Instead of raising it, I fitted lower profile tyres (were 80,now 65),problen solved! And they match the rear now. Vibrations way way reduced, suspect the original fronts were not balanced anymore.
Carbs are now balanced, but rich at the upper end of the rev range so I get quite pleasing but anti social pops on overrun. I've some less aggressive needles to put in, see how that goes.
Now I'm using it, rain or shine, for the weekly shop and it's just brilliant.
Thanks everyone for the help with my endless questions, it's on the road mostly due to advice from this forum.
A far-too-long video of the first drive after tuning up with the side exit.