I'm currently doing a diploma in clinical community pharmacy at Aston (
PgDip/MSc in Clinical Community Pharmacy). It's tough going at times, but I've learnt a lot and the material is well written - some bits were written by people at NPC for example. You can always email Kay Wood (k.m.g.wood@aston.ac.uk) and ask her to send you a sample of the course material.
I think the course will benefit me in the long term - it's not so much what you learn as how you learn to think. It will also open up more doors. I don't think there's much of a role for PwSI at the moment, but then they are fairly new - look at how long it has taken MURs to take off.
Doing a clinical diploma won't hurt you in the long term, and should be a huge benefit (there aren't many community pharmacists with clinical diplomas after all). You should also be able to get your employer to pay for the course, and if you do the aston course you get two residential weeks in birmingham.