As tall as they like. There is no 'right to light' or right not to be overshadowed. The only exception, where the law was changed a few years ago, was to give certain powers where high Leylandii hedges are concerned.
The only thing you can do is to try to persuade your neighbour to pollard the tree if it is a suitable species. Otherwise, I am afraid you will have to tolerate it.
If it actually constitutes a legal DANGER because it is causing subsidence or is likely to fall or drop branches on to your property, you could try getting a professional tree surgeon's report on the tree (the local council's tree officer might help) and then copy it to your neighbour with a solicitor's letter pointing out the liability.

i agree sorry .

Whilst you may be unhappy with the tree unless it is a hazard it is very much needed as a home for birds and insects. Is this the same neighbour who's garden you say is a 'mess'?