Statute of limitations is 3 years but that is only to file a claim with a court. Once that is done it can drag out as long as necessary. I didn't settle with the guy who drove up the back of me in 2004 until last year, that's 5 years to a settlement.
When they asked for £400 you should have got them to sign a statement that they have received £400 in full and final settlement for the incident, and frankly you should have bitten their arm off.
Since you refused to settle amicably they have reported their claim to their own insurance who will have given them a hire vehicle (you say this was a van, so it will likely have been a Transit or a Caddy or similar commercial vehicle) while her van was repaired. Then the lawyer will have gone down the personal injury route, which is particularly tempting as the MIB don't pay out the full amount so a personal injury claim is a great way to make up the difference.
The MIB pay all this out and when they finally track you down they demand reimbursement as you are now finding out.
Clare is ****ed. I hope your brother's financial arrangements are such that he isn't deemed "financially linked"* to his GF, else your brother can be held liable.
*eg. they have a joint account. Bills in both names. etc.
I know this process well as my GF's first 200SX was written off by an uninsured woman in a Vitara, so we've been through it from the other end. We didn't make a false personal injury claim though.