Thanks
I've now used a small piece of broken paving stone that I had lying
around. The hood's old so doubt I could find the part anywhere on line
(I've tried looking for the hood online without sucess. I guess the
builders just forgot to put the cover on.

Val

Andy Pandy wrote:

If it's vented to the outside from the back of the unit, the top hole
should be covered with a circular plastic cover. It only there as an
alternative route and can be blocked up..

Andy