Hi Stuie,
welcome to the forum!

Need a bit more info, does the knocking sound happen with the engine on or off, moving or stationary??

It could be one of the injectors? I think they normally make a ticking noise which some people have mistaken for a fault.

Im guessing others will have more answers but as majority of the forum members are in the USA they dont frequent the forum at the same time as us Brits. If you dont get many views here in the UK section you could post it on the main forum.

Good luck
Matt