Its a ceramic disc tap so you have to replace the whole cartridge if you do not know the name of the taps this will be impossible I would just opt for new tap Imho
You are not gas safe registered at the present time so you should not be working on gas apliances and these are very dangerous boilers if not inspected and comishioned correctly LEAVE ALONE
http://cgi.ebay.co.uk/Draper-Immersion-Heater-Box-Spanner-3-3%2f8%22-AF-2-1%2f4%22BSP_W0QQitemZ370427532631QQcmdZViewItem?rvr_id=150755365458&rvr_id=150755365458&cguid=45f921e812b0a0e201c75fd7ff04647c
Try this it should be deep enough :D