(1) In so far as the defence of change of position is still nascent, and it is not yet crystal clear (in light of the dearth of decided cases) that the change of position would cover the example, estoppel might still have a role to play. In fact, a litigant may justifiably prefer to go for the more certain defence of estoppel.

(2) Even if the change of position defence is such that it is available whenever estoppel is, change of position only allows D in my example to keep what estoppel also allows him to keep. It cannot, contrary to what is claimed by the 'novel' argument, knock out all of D's detriment.

Illustration:

In the example, what if the print is now worth £100 in the market (or to avoid causation difficulties the lost opportunity can be an investment in a fund with guaranteed returns). D would suffer at least a loss of £50. Even if change of position applies so D can keep all of £100, he would still be left with a detriment of £30, since his total loss (£50 from the increased price and £80 from the deposit) exceeds the mistaken payment. The change of position defence would still not knock out all his detriment.

Anyway, this is just a minor technical point on the argument. Whether estoppel and change of position should merge should rest on wider, policy considerations, it seems.

Lusina Ho