I rang EMG Croydon for you, they still look after Rover and sell used ones, nice helpful chap on the phone as wellBattersea seems a long way to go (but you might be going past anyway). I have read here that Keen's are good.
According to http://www.the75andztclub.co.uk/index.php?thepage=dealer_surrey
EMG who are in Epsom (and I was going to suggest) also have a branch in Croydon:
Looks like they've moved! but this link is today's...
17 Montague Road
Tel: 020 8688 8341
19 Blenheim Road
Longmead Business Park
Telephone: 01372 739 900