Mosquito Dunks or Similar for pond (Willesden NW2)

I have a very very small pond in my garden, probably only has around 50litres of water if that, been covered over for around 2 years ( took care of a dog for a while). Can hear very audible buzzing from under the cover, i am quite sure. Had a mozzy problem when it was not covered also. Does anybody have 1 or maybe 2 mosquito dunks to spare. Will empty pond after and fill. Would be very very gratefully received. Dont mind travelling either, close to Willesden would be great. Thankyou so much in advance