TCV's do fail with age. So maybe it is as simple as that. I know Pick and Pull type yards are getting rare in the UK, but we still have them here and I usually keep spare from the yards just in case.

Neil.

That's 20mpg, which is completely normal for a P80 HPT. Good even, with that split city/highway.

Disregard boost in 1st gear. 12-15psi is very healthy.

Without the back story and codes, I'd give this engine a high mark for health. I haven't looked up those codes but will.

I have a 2000 V70R with a tune running a claimed 310bhp (tune from Rica in Holland). It is on the standard CBV and it has up to 20psi boost.

Neil.

I have the Forge unit on my '96. It's a work of art. Do you need it ? Probably not. But it is nice to have, and changing springs is easy if your needs change.
I'm not sure about a BOV. I've heard it is more for sticks than autos.

The stock CBV and a BOV are two different functions. The stock valve pulls off boost when you lift, the intake manifold vacuum sucks the valve open and reduces boost in this case. You can buy some stiffer springs which will allow more boost but IPD warns you that the red spring without a tune will m...

You have a bunch of communication issues! I would recommend running a smoke test on the intake manifold to find any leaks. There are YouTube videos with instructions for building a smoke generator with a soldering iron, glass jar, old sock, and baby oil. The smoke test saves a ton of guesswork and i...

DEM error is either bad pressure+temperature sensor or dead pump. Pump lasts about 10-11 years. Can be a bad DEM, too.

"was hoping the EVAP stuff would solve the fuel odour in front cabin too but no luck there." Check for fuel leaks from the upper o-rings on the fuel injectors. You should be able to see any fuel wetness after removing the small black cover with the two T30 fasteners. Also check for a leak ...