I will try help with you question.
Has there been any change in the legal ownership since your solicitor first told you that it was 189 years and do you have a written record of your solicitor informing you the lease was 189 years?
Subject to you having the written evidence (though this is not fatal to your claim) and there being no legal changes since that time, then it sounds like you have a case in negligence against your solicitor. Your solicitor owes you a duty of care which has been breached and has caused you a loss (as the value of your property is likely to be less than you anticipated based on his/her advice). You may also have a case for breach of contract as all contracts imply a term requiring the work to be undertaken with reasonable skill and care.
It will be easier for you to claim in negligence but before that I would advise setting out your position to your solicitor in a letter and asking them what they will do to rectify it. Give them a deadline to respond (say 14 working days) and state that should you not receive a satisfactory response and offer to correct the loss then you will be forced to lodge a claim against them in the High Court. I would suggest that they should be commissioning a valuation at their cost to determine the difference in value between 189 years and 126 years and they should be compensating you for this amount
Hope this helps