[fizzwheel]

In my experience Donnington is not the best for photos, the two layers of catch fencing cause issues as does the fact that you are in places stood alony way from the bikes themselves for obvious safety reasons.

At 140 - 150mph a bike can travel along way in 0.3 seconds, you need to work on your technique, so that you pan with the bike as its moving so keep the camera focused / pointed at your subject matter.

The issues with poor quality pics when enlarged to print arent being caused by megapixels, 4mp should be enough for a decent A4 print. Its more than likely the way the camera is zooming. I.e its using digital zoom rather than using the lens to get in close. You loose quality with digital zoom...

Sadly you arent going to stretch to a DSLR on your budget, you might want to look at a Bridge camera, which sits between the compact and DSLR, you get more features such as control of the shutter speed etc etc and I suspect it'll have less shutter lag as well.