[Avulon]

Witnesses are definitely a must and the one thing that you need to get at the time of the accident name/address/phone number or even just registration of other vehicles that stop and are possible witnesses (the drivers that is): Are there any CCTV cameras that might have recorded the event? get on to them quick before the tapes are overwritten and get a copy.
Second:don't claim on your insurance, period. Doesn't matter if you are fully comp - that's not what fully comp insurance is for (it's for when you crash on your own and it's your fault). Of course fully comp will cover you but not really any faster than the other person's insurance (provided they have some). Claim directly from the other driver's insurance. Yes you do have to inform your insurance co. Make it clear that this is a report only (as they no doubt will require) and a not a claim. Keep posession of your vehicle - have it recovered to safe place under your control (prefer your own home driveway/garage). Insist on independant damage assessors. And under no circumstance talk to any 'claims handling' companies!

Keep a written log of everything that happens from hereon -names, dates, times what was said if not written down, all correspondence and email with anything remotely connected to the accident. If the police attended then request any records they may have (might be none at all - honestly!) including the attending policemans notebook notes (if he bothered to make any: make sure you do have his/her number and operations base).

Third: consider getting your own solicitor to handle it.

P.S all this advice is from recent personal experiences.

On another note, I hope no one was injured - but do make sure to get any pains checked out even if you think that you are ok. (and keep records)