If the email contract was to sell the bike, listed for £X originally, at £X-1, to buyer A, and buyer A failed o honour that offer, eg didn’t pay in time or didn’t pay the correct amount, then no contract has been made. You are under no obligation to seller A as they have not honoured the full offer they made with you and so therefore havn’t made adequate acceptance.
Buyer B however has seen your offer, £X, has paid £X and has therefore accepted your offer.
I’d say you are obliged to honour the contract with buyer B.
Oh, buyer A has been p155ing you about too by the way trying to lower the offer price ang seemingly request unreasonable delivery demands.
Sell to B. Give A his £10 back saying £10 was not enough to buy the bike.