I wrote an answer to my own question yesterday, as I have since resolved the issue, but it's disappeared - or maybe I did something wrong whilst trying to post it. Anyway, I thought I'd post here in case it's of use to somebody else who may have the same problem.
A friend of a work colleague (who works in this industry) suggested it may be because the plastic lining that juts out from the back of the condenser unit had become mis-shappen or distorted, and I should try cutting off a piece of the plastic so that it slots into place again (this is only about 1/2 cm long and 1/8 - 1/4 cm thick). Before resorting to cutting, I just worked the plastic with my hands, pressing and manipulating it 'inwards' to try and make it contract and shrink back to size, as it were. I'm glad to say this did the trick and it now fits again. Simple when you know how! :)
TammyP
November 2012