How can consideration be past when the offeror demanded a price, and all discussion of consideration precedes the contract signing?

Question

How does the issue of past consideration arise at all? I'm relying on the paragraphs chronologically. Obviously P's acceptance (see bold below) precedes 'P and D signed a contract'. Then any consideration from D (for P's promise to sell the bicycle) must hail from the £50 that P's demanding. I.e. 'in consideration of the excellent work' can't form any part of consideration, because P's also demanding £50 from D.

Here's the problem question.

Plaintiff decided to sell his bicycle. He advertises as follows:

FOR SALE
Bicycle £49 (price negotiable)
Please call: 020 1234 5678 to accept this offer – first come, first served.

Defendant, a mechanic, had always looked after P’s bicycles. D wanted to buy this bicycle but the most he could scrape together was £25. He rang P to ask whether P would consider accepting £25 now and the balance over the next five years at £5 per year. P said ‘the price is £50, but in consideration of the excellent work you have always carried out on my bicycles, I accept your proposal’ [Call this P's Acceptance].

P and D signed a contract detailing the arrangement and D said D would collect the bicycle the following day. Later that day P changed his mind and contacted D to say that D couldn't have the bicycle unless D paid the full price immediately.

D believes he has the right to the bicycle in accordance with the signed agreement. But P argues that
1. there was not sufficient consideration for D's promise, and
2. even if there was sufficient consideration, the variation promise was not binding on P.

Here's the relevant part of my textbook's answer.

Was the consideration from D sufficient? P’s statement “in consideration of the excellent work you have always carried out on my bicycles, I accept your proposal” suggests that P would be using past consideration to support the price variation. P's statement looks like past consideration (Lampleigh, Pao On) and therefore not sufficient. So no contract to vary.

Answer 1

I would say that when P accepts D's offer, and P and D sign a contract, that P and D have formed a contract. The consideration for P is the £25 at once plus the promise of £5 per year until £50 is paid. The consideration for D is the bicycle. As P advertised that the price was negotiable it was reasonable for D to offer this deal, and P could chose to accept or not. Having accepted unconditionally P cannot undo the deal without D's consent. If D had been a company, consumer protection law might give P a period within which P had a right to cancel, but that would not apply between two private individuals. P is taking into account past good will in accepting a lesser consideration than P's advertisement asked, but that is not really consideration for the contract, just a factor influencing P's decision to accept the offer.