Presuming you have an IR aswell....
1. Get a Class 1 medical from Gatwick.
2. Do the 14 ATPL exams. Bristol have a good reputation for groundschool, although I used CATS and they were fine. Bristol's brush-up residentials are a little more constraining for those with demanding jobs.
3. Training as required, then CPL test (£750).
4. 15hrs minimum conversion training (probably more), then IR test (another £750).
Plenty more detailed info if you use the search facility...