Just a quick update to this thread.
I bought a new diverter valve from Awesome GTI, it arrived a couple of weeks back but I've not had a chance to fit it until now.
I replaced the valve this morning, took about 30 minutes tops with the car jacked up, much easier to replace than I was expecting.
I was a little surprised when I removed the old valve to find that it appeared to be absolutely fine. I had a good look at it and the diaphragm has no splits or tears on it.
As I had a new valve anyway I stuck it on and buttoned everything up as there's no point in returning it when it was only £40 or so.
I've just got back after heading out in the car with the mountain bike.
All I can say is bl**dy hell it goes well now
It's like driving a different car, I thought that it was just the way the car was meant to be previously, seemed a bit flat but I just got on with it.
The difference with the new valve fitted is incredible, it's like it's got an extra 50bhp!
It's fair to say that after thinking the car was only so-so previously, I love it now it's going properly
The only slight problem now is that after having it jacked up I've started noticing a bit of a knock from the front suspension so it looks like that'll be getting fixed next!