It is much easier these days, especially using "tell us once" online facility, to apply for probate and complete IHT forms etc. If you can do it for yourself it will save you money. Most organisations now have dedicated bereavement teams and once registered, they handle all queries on dedicated phone numbers so you don't get messed around as much as used to happen. If you have your own solicitor, they can provide assistance but if your late mother's affairs are not complex, you should be able to complete the process without too much help from them.
You also have this forum to help you if needed.