Hi Bill
Yes I did find it in the end, took all the stuff out the front box and there was a little pipe like you guys described. I think it may be rusted but good to have something to work with, I plan to do some more exploring when I have a chance but it looks about 8mm, very rusty on the outside so makes me wonder if the inside is the same. I still want to try what you suggested to see if water comes out of it then we will know it’s not blocked. Its re-assuring to hear other peoples tanks ‘boom’ it either when we put water in or when someone is using the shower. Hopefully I can have a proper look at it this weekend, would be great if it’s just blocked, we shall see. . .
Thanks again for all your help!
Kassia