It happens that fred formulated :
We have a drain at corner of the house which consistently gets blocked with
seeds from surrounding trees. In the past Ive placed one of these
https://www.screwfix.com/p/floplast-...mm-black/78832 , not that
exact model but that type. First problem was wind whipping it away and second
problem was it was too low. The down pipe is cut quite short so it doesn't
engage with the entry hole in the guard. I can live with this problem or just
extend the down pipe . So I need some simple way of fixing the guard to the
tarmac (or what ever the correct name is). Haven't bought the new guard so
would welcome alternative solutions.

I just put a spare brick on top of mine. I think I paid £1 each for the
guards in a pound shop.