KJT:
The miss could be a number of things... Check you plugs, leads first.
The MAF is the Air flow meter -give the ECU info about the air gussing down the air intake. Sits just after the Air filter box and hsould have a multi plug on its right hand side (looking from front to back). Unlplug this and go for a spin. If no difference then this may be your problom. Logic here is that if somthing that should make a difference is unplugged and doesn't - then it be fu**ed.
The 4K rev limit may be the MAF aswell plus check that the Air box is clear and flter isn't blocked.
You could always try spaying a can of carb clearer down the air intake infront of the MAF whilst on idle. This will clean or go some way to cleaning the MAF and might just sort you out before you spend £50 on a new unit.