Hugh wrote:

Not a d-i-y answer, but have you tried your local council?

Good point, which reminds me. If it's a railway wall, try the railway.
They greatly dislike having stuff painted, stuck or fixed to their
property and may get around to shifting it. They'd probably disapprove
of your attacking the wall with paint strippers or pressure washers, if
they found out. Might be quickest to DIY rather than await their
efforts though.