If the auction still has days to run, then no you don't have to sell at the £20 'offered'.
That is just an initial bid, it may, hopefully, change over the remaining period of auction as and when others decide to enter the auction.
(I also think you mean £55 is the 'Buy It Now' price where, if somebody does offer you £55 you have to make a decision to sell).
If there is no movement over £20 at the end of the auction, then come back to us.