If your mother took out 'legal expenses' cover, on top of her basic premium, she should contact the insurer to get them to pursue her claim against the other driver. (NB: Although your mother might have paid a 'combined policy'' price to get legal expenses insurance added to her car policy, the actual insurer will almost certainly be a different company to the one that the car was insured with. She should check the documentation).
If your mother did not have 'legal expenses' cover, she can still pursue a claim against the other driver. She should list all of her losses (e.g. "expected cost of replacement vehicle minus insurance payout received", "public transport costs", etc). She should then send the other driver a formal letter, outlining those costs and demanding payment within a reasonable period (of, say, 28 days). The letter should also indicate that, if payment isn't received by the relevant date, your mother will pursue the matter through court action.
If no payment is received from the other driver, your mother should initiate court action:
http://www.hmcourts-service.gov.uk/infoabout/c laims/index.htm
Chris