Your Mum will recieve a payout from the MIB, it doesn't make any difference the other driver was uninsured for practical purposes.
http://www.mib.org.uk/MIB/en/Claims/UK/Uninsured/Default.htm
MIB was established in 1946 as a private company limited by guarantee for the purpose of entering into agreements with the Government to compensate the victims of negligent uninsured and untraced motorists.
Every insurer underwriting compulsory motor insurance is obliged, by virtue of the Road Traffic Act 1988, to be a member of MIB and to contribute to its funding.