Somebody with more knowledge will give more information, but just to give you the basic facts. Once the will has gone through probate it becomes a public document and anyone can get a copy of it from various places. Before that if you think you have a claim for money from a will I think the executor must deal with the claim correctly or the probate office could become involved.
What colin says is correct. The fact that you have received a cheque should mean that probate has now been granted so you can apply for a copy of the will. Go to the probate registry website, which should tell you the procedure for getting it.
As has already been stated, once probate has been granted a will becomes a public document and anyone may consult it. There's a search fee involved but it's only �5: http://www.hmcourts-service.gov.uk/cms/1226.ht m
The solicitor dealing with your father's estate should supply you with a copy of the will and a breakdown of the administration of the funds, you need to ask if they have not sent you one.