TBH I spoke to my brother, and said that I wanted to have a go at Probate, as to pay thousands for someone else to put in order the paperwork that I would have to find anyway, seemed ridiculous.
I started by preparing a list of assets (no property, but bank accounts etc.), and a list of everything she still needed to pay for. This included funeral costs, catering, re-fund overpaid pension etc. This information provided the basis for the whole application.
I wrote to all of the building societies/banks she had accounts with, and they provided the up to date figures - they needed a copy of the death certificate to do this.
There is guidance on the HMRC Probate bit of the website, but I did fill in the incorrect forms once (I think it is slightly confusing), but called the Probate Office and they talked me through it.
They made the error of calling my brother and I in to 'swear the Oath' before HMRC had approved the application, which caused a bit of a confusion, but we got over it, and didn't have to go in again.
Just one point, passing stuff to her son doesn't make the estate inheritance tax free - although it may be possible to take into account any unused allowance from her late husband's estate (if that applies).
It actually gave me something practical to do, which I think helped me come to terms with Mum's death.
I would strongly advise that you take legal advice on the house though, to make sure that your MIL's wishes are adhered to.
Good luck!