John - glad to hear this seems to have solved it for you, and happy to know it's not just me to have experienced it!
Naz - it's called an anti shudder valve. If you give it a quick search you'll find quite a lot about it, but more often related to VAG cars. There are pictures of location on the previous page, but it's not something you'd find on a petrol car though (unless I'm very much mistaken).
It's too dark now, but I'll take a picture for you tomorrow as it seems like the easiest way to help identify it.
In very basic terms it will be the largest diameter hose (turbo hose) that comes up from the bottom of the engine to the front near the top.
If you loosen the clip at the top and slide the hose off then you should be able to reach the butterfly valve with your hand.
Try closing it by hand and seeing if it sticks at all or is particularly tight.
I also found that squeezing the turbo hose would sometimes be enough to release the valve if it was stuck. In this case listen carefully to see if you can hear it releasing. It should always be open apart from within a few moments of the engine stopping.
I never received an engine management light or fault though, so it's entirely possible you have a separate issue. Here's hoping it's simple!