It turns out that the solution was embarrassingly simple
Basically, the symptom I had was that the car was bogging down and losing power when the throttle was applied. I tried everything... Stripped, cleaned and rebuilt the carb. I replaced the carb to manifold gasket. I found a split in the vacuum hose to the dizzy, which I had missed when I'd previously set the carb up, so I had to replace this then re-adjust the other bum droppings that I'd adjusted to compensate the vac leak. I removed all the electrical sensors and tested them, checked all the wiring for them as well.One thing I did not check was this thing (circled red)
It's a vacuum diaphragm attached to a flap basically. For anyone interested, at low temperatures the air being drawn through the carburetor can cause ice to form in the fuel jets. This doesn't actually have to be air temperatures below freezing, as the air will drop in temperature as it is drawn from the relatively large space of the air box into the throttle channels. The flap in question is vacuum operated and controlled by a mechanical temperature sensor (green circle) in the air box which operates as a vacuum 'switch' and opens the warm air flap. This prevents cold air coming from the cold air feed (not fitted on my car as it was made from foam and disintegrated
) and instead draws air from the warm air feed (silver tube) which is connected to an air collector over the exhaust manifold. This in turn sends hot air through the carb preventing icing. 
So, seeing as my problem did seem like carb icing, I decided to finally check this mechanism. Turns out it was seized shut in the position that would make it draw cold air
Luckily I have a few spare air boxes so I checked one was working, lumped it on, and happy days it all works now 
I've been out for a good hour or so in the car and it was all good, bar the idle needs adjusting and I need to get the idle CO set properly. Both easy jobs luckily 
At any rate, I actually enjoyed driving this car for the first time in ages
I'm definitely feeling more inclined to get all the bodywork issues sorted now.
Plan was to just get the exhaust manifold from an 8v GTI and spend even more time fault finding/setting up the carb.
). Watch this space 
