Not only do you have to repay some (most) of your lump, if you join ftrs within a year (or possibly two) but they take into account what "they" are paying you in pension and deduct that from the pay for the new job.
So don't go thinking that you can retire, spend the lump and live off the monthly money, then get a job at the same pay as you were on before, that's not how it works.