Hi Louis, what happened to flying the PA28?
With regard to approach techniques, hmm, you've not been doing it long, so you cant expect perfection. If you are worried you really should have a chat with your instructor about it. You said you tend to undershoot, is this on RW22, or 04? If it's 22L remember 800 ft over the pylons, 500 ft over the M25 should set you up nicely. On 04 if you are undershooting, use a bit more power, you have to think ahead and realise its happening early (usually if the aiming point starts raising in the windscreen). If you are left or right of the centre line, it would suggest to me that you are not correcting for wind correctly. It really is down to anticipation and practice. Dont beat yourself up over it, its part of learning to fly!
Talk it over with your instructor (they appreciate a nice cup of tea), and if you are still not happy, ask for a lesson with another, its your money after all!
Best wishes,
Iain.