Hi pal.

I tried it without the cap and it was the same.

I then tried it disconnecting the breather and it ran like new again.

The breather pipe on the top of the tank runs to the breather which is mounted on the fuel filler pipe in the passenger wheel arch (or anti-trickle fill valve as I have found out it is called) Then out of that there is a pipe that runs the full length of the car to a charcoal canister (evaporative emissions control carbon canister.) where it stores the vapours. Then there is a canister-purge solenoid which then puts the vapours into the inlet tract to be burned by the engine during normal combustion.

So the problem looks to be from the breather to that canister. The fuel tank has also imploded again

I'm not too fussed as I'm just glad the car is running right again. I will figure it all out. Respect to the Haynes Manual. Been a nightmare weekend though