Triangle headed locks are widely used in street lighting and street furniture. This company supplies a wide range of keys.
https://www.pudseydiamond.com/keys/tri-head-keysand their chart might help you find the right size.
Each batch of street lighting columns comes with a key or a number of keys so local authority lighting departments tend to have a ready supply and don't buy keys separately very often.
CEF sell one size of key
https://www.cef.co.uk/catalogue/prod...ghting-columns and the branches near to you seem to keep some in stock. If you took the clock there, you would be able to check the size.
PMM