Hi
Could you say what your subject/discipline is as that might help. Have you considered the open university for p/t teaching - they are recruiting across the UK, but again, it depends on your subject.
Some departments advertise for p.time staff at a hourly rate - have you checked the websites of local colleges (I'm sure you have!).
As for writing a letter - yes, it is worth doing so, though you may still have to fill in an application form if they are interested. It is also worth ringing the Department Head to check of any p/time pool they hold.
The letter itself: not that different form one for a perm. job. Ensure the letter gets to the right person - and name him/her on the letter. Stress what you can teach (i.e. read their course material), what you have taught and methods used (e.g. tutorials/seminars). It is also worth briefly illuminating your research area in ways that can show a positive contribution to teaching. Enclose a c.v.
Websites: have you tried
www.grad.ac.UK? I also think Kent university have some c.v./letter samples on their site.
If you can say More about your subject others might be able to help more?
Best
Sue
