Thankyou both for your kind words of support, my Gerbil has had the operation to remove the cancer (and his penis!!) he is recovering well after his big panic attack on Thursday evening on realising what had just happened! he has to return to the vet next Saturday to have his stitches removed, until then i just have to make sure he pees regularly. he is not drinking much so i have been giving him fresh fruit/veg rather than dried food which seems to be working! the vet will clip his teeth in Saturday too as they are not being worn down (i have taken all his chew toys out of the cage so he doesn't catch his wound on them or get the urge to jump over them!) he is enjoying all the snuggles he is getting and began 'chating' to me again today which is a great sign that he is not hurting. As for his brother, the vet is happy to leave him be for the time being and see if it develops, if it does then he will operate, if it stays as it is then i think it best to leave it alone as it is not bothering him nor does it look sore.
so all in all a good result and the right decision made.
Thankyou both again x