This is very much a long shot, but you could get a copy of the house title deeds from the online land registry. This will show you who the actual owner(s) is/are. (I very much doubt whether the police checked this before giving the partner the keys.)
If the house is in the partner's sole name then I don't think there is anything you can do about getting access. If it is in joint names as joint tenants the same probably applies, because the survivor of joint tenants automatically inherits the deceased's share.
But if it is in joint names as tenants in common then your mother's share will be disposed of in accordance with her will or - if she did not make one - under intestacy rules. You would then have an argument that the partner may well not be the sole living proprietor & it should be possible to bring some pressure to bear on the partner (through his solicitor?) to release the keys. Alternatively, you could take legal advice on whether - if he still refused - you would be justified in forcing entry.