Wot-a-pong

Ive a Twyfords hand basin , with a cast in overflow, the problem is
cleaning ‘inside’ the overflow , I used to use a foaming bleach
(harpic) and simply fill the 'chamber' but that’s no longer on
sale ,

So sometimes, after a warm period or the heating has been on I get a
fowl smell from the overflow , ordinary cleaners don’t have much
affect, last time I used cling-film over the waste trap and flooded
the whole sink with hot water and bleach , that solved the problem for
a while , but its finally returned … bit messey, had to rip the film
then to drain the sink

Any ideas on a cleaning product or way of dealing with it ?

Tnx – G ..