Iain it is usually best to choose components based on what you want them to achieve, i.e. why do you want to change and what new functionality if any is desired and at what price?
If your bike has the same spec as
this, then unless you are changing your brifters (which will be costly) your new cassette should be 8 speed, which means any new chain should ideally be an 8 speed one.
The choice of new rear mech is somewhat dependent on the cassette and chainset specs, which are dependent on whether you are happy with the existing gear ratios. If the existing configurations are to be retained I think it is best if you stick with a mtb long cage rear mech from Shimano, otherwise something else might suffice.
Truvativ Isoflow chainsets come in two flavours: their proprietary power spline or square tapered. If you want to change it you need to figure out which one yours is unless you are to replace the bottom bracket at the same time, either way knowing and retaining the chainline can be important. If you want to change the front mech then its selection is also somewhat driven by the chainset's spec.
Drivetrain parts mentioned so far can cost anything from around £100 to well over £1000 in total.
As you can probably gather, knowing what is wrong currently, what you wish to ultimately achieve and at what budget will be necessary for an informed choice.
