Just to set a few things straight here so you don't go saying things to them that aren't true.
1. They do NOT have to give you the salvage option. You can request it, but they do not have to give it.
2. That £3000 value you put on it is not an insured value. All it does is gives them an idea of whether it needs to go to an underwriter to be quoted on at the time of giving a quote for cover. The only time that you have any figure to go back to is if you have specifically been through the agreed value process.
Given that 99% of people want to get a claim sorted as quickly as possible, the insurer has done the best it can to make it like that. They have started the process by sending the cheque and arranging to have the car recovered. This offer can be rejected if that is what you want to do.
They haven't actually done anything wrong, they do not legally need to send anyone to look at the car. They can make a decision from whatever information they have.
You cannot call the insurer unless your mother has specifically given them permission to deal with you. To do so would go against the Data Protection Act.
You need to get her to call them and give them permission to deal with you if that is what she wants. You can then talk to them, but I'd do it much more calmly than you have come over in this thread or you will get nowhere with them.
If you think they have under valued it, you will need to be able to validate this. Examples currently for sale is one method. You need to present to them this evidence if request with your valuation of the car. You can also request that they take into consideration you keeping the car within that price. They may or may not allow this. If it is deemed a Cat A or B write off, then legally they cannot offer this to you and it must be taken away and crushed. A classic Mini with rear end damage to the chassis could well fall into this category.
Hopefully that puts things into perspective. They are trying to do the right thing by what the vast majority of people would want.
