Checked my air filter last night and noticed it was clogged with dust. OK I did get stuck going up one of the slopes and did create a lot of dust from the front wheels.
Where I'm getting to is that the common air filter for Fronty's (HOF328) is listed at £12.98 - with using my trade card I paid £5.28 or thereabouts (haven't got the recipe with me), but if anybody else needs one for that money, let me know and I'll see what I can do.