Do get a copy. Your situation encapsulates the issues faced by many blended families. If your Dad's wife has inherited the lot, then, as you say, she is free to do with it as she would like. I am not sure what your grounds for contesting the will would be. I would suggest you contact a solicitor (once you are in full possession of the facts and NOT before) and one that will give you 30/60 mins of free advice.
Here's how you get a copy of the will:
Assuming that probate has been granted, you can visit First Avenue House, 42-49 High Holborn, London WC1 6NP (tel: 020 7947 6939) which is the principal probate registry for England and Wales, and make a search in person.
Alternatively, you can visit the probate registry that covers your district.
There are also 18 sub-registries and you'll find a full list of both with addresses and opening times on the HM Courts Service website.
It is possible also to request a copy of a will by writing to The Postal Searches & Copies Dept, York Probate Sub-Registry, 1st Floor, Castle Chambers, Clifford Street, York, YO1 9RG, giving the full name, address and date of death of the deceased, stating what you require and enclosing the £5 fee.