I have accrued 11 weeks holiday pay over the last Four years , can I cash this money in rather than take the leave.

That is up to your company and their printed policies. Do you have an employee handbook? My employer allowed me to either cash in my saved days or to retire early and keep getting paid until the accrued days ran out. Then again, I was the HR director at the time so maybe I got off easy

In many companies no, and it is "paid time off" or "vacation" time, holiday pay is pay you get when you take a holiday off