Firstly, if I can correct a couple of fundamental misunderstandings.
"As long as she was NOT suffering from dementia when she made the will you do not stand a chance." - incorrect. The absence of dementia does not lead to the conclusion that the deceased had testamentary capacity. The full test is set out in Banks v Goodfellow and it is not quite as simple as "did she have dementia or not". There are also other ways of challenging a will.
"Unless you were financially dependent on your mother at the time of her death, you have very little chance in successfully contesting the will" - incorrect. A child of the deceased does not have to establish dependency to make a claim under the Inheritance (Provision for Family and Dependants) Act 1975. Granted, lack of dependence makes a claim harder, but that is not a requirement under the act.
"If you can prove that you worked for a period for almost nothing except a hope of an inheritance, you may have a chance. But you'd have to have strong witnesses to that effect, or a signed letter or contract. " Getting closer! What the poster is alluding to here is the doctrine of proprietary estoppel. For a recent example, see Thorner v Major. Essentially, if A is promised something by B and on reliance on that promise acts to their detriment, B is prevented from acting on their strict legal rights and will hold the promised property on trust for A. A signed contract is specifically NOT required.
GB1959 - I urge you to take legal advice from a solicitor who is a member of STEP or ACTAPS or a barrister who is a member of the Chancery Bar Association who has direct access. I am not saying you DO have a claim, I am saying it is worth you getting proper legal advice to know definitively since there is an awful lot of incorrect advice on this thread. You have said enough to make me think this needs further investigation.