sassenach512. What you need here, and urgently is good legal advice. I would advise you to visit a solicitor (one who specialises in wills and probate) as soon as possible. Present him with the facts of the case and take his advice. If you really cannot afford that, then go and speak to your local Citizens Advice office.
You have not said how much money you received 18 years ago (and I am not askng), but was it sufficient to amount to at least half the value of the house at the time of your father's death and possibly more if your father had savings and investments? If not then more is owed to you, especially as, as the house is only being sold now it will be worth more and you will be entitled to half the increase in value.
In my opinion, your brother is trying to cheat you out of money you are entitled to. Because, when your father died, for good reason, you didn't challenge him then, he thinks he can slip a fast one past you again. Do not sign anything without letting a solicitor have a good look at it first. You need a solicitor to watch over your interests during the probate application, who is quite separate from the one instructed to handle the probate by your brother.
You say of your brother he was always a controlling older brother, I just let it go. If a solicitor deals with everything for you, you will not have to speak or deal directly with your brother at all. Your solicitor can protect you and ensure you get whatever you are legally due.
If you are nervous about about visiting a solicitor do you have a partner, child, or friend who could come with you to the first meeting and support you through the process?
Your brother has clearly never applied for probate. There must be serious questions about what other savings and investments your father had when he died and how, if at all, your brother managed to get them released to him when he did not have probate
Solicitors do not usually charge fees until the job is complete and you could be due some serious money, running into 10s of thousands of pounds.