Beko slimline dishwasher DE2541?

The hot air drying, round plastic disc at the top right hand side of my dishwasher keeps falling off! It can be put back on but every time I do a wash it falls off again. The disc doesn't appear to be damaged in any way and I can't see why it won't stay put!

Can anyone help please, I don't want to pay an engineer £100 to tell me there's an easy way to fix it back on permanently.

Hann
August 2012