I'm assuming you're in Britain
If so, you could try contacting the Coroner and seeing if he or she will help.
The insurance company are playing for time and it's possible that a sympathetic Coroner will stick an oar in on your behalf.

When we traumatically lost someone close the Coroner and his staff were very helpful and ready with advice. They understood how hard things can be for the bereaved.

In your circs, Jess' mum is entitled to that support too. Worth a try.
(Juan Quar is clinging to her front teeth on, Tue 22 Jun 2010, 8:52, 1 reply)

^^
This.

Also, I would do 2 further things:

1 - phone up Arse Invaded Goats (see what I did there?) and demand that they show you, written down, where an interim death certificate is not considered proof. It has a written cause of death on it.

2 - this is a good case to take to the local papers. I know it may seem distasteful, but it would very much help. If not, Consumer Direct or even Watchdog may assist.
( Undercovercarrot got done for gross misconduct on..., Tue 22 Jun 2010, 9:06, closed)

yup
there's no such thing as bad publicity. However, there is such a thing as very bad publicity.

Tasteless it may well be, but they deserve it.
( sandettie light vessel automatic New Twitter - @bollocksreally, Tue 22 Jun 2010, 9:34, closed)