I have got the same problem with my boiler, the E20 pops up. I don't know if this is of much help to you:
http://www.potterton.co.uk/docs/Pot_gold_combi18HEA_Users_sept_10p.pdf
On Pg.8 it basically says, 'E20 Central Heating NTC Fault'.
Awaiting input ...
Thanks.
Help
January 2011