I recently received a letter from a solicitor stating my ex partner wants to see our 2 children . He has had no contact with them for a year and a half and has never paid for them and also is into drugs is there a chance he could be granted access

Yes. If he went to a solicitor he may have gotten himself clean. You can ask for supervised visitation.

Yes a very good chance, fathers normally have rights to see their children. You can request these visits be supervised at least to start with. If you have proof of drug use in past maybe even require drug testing.