Interestingly, this is something I've meant to ask on FT! There's a minor checked bag dilemma coming up:

I'm flying from TXL to LHR on 31 Dec, spending New Year's in London and flying out to BOM on the morning of the 1st (!) in F. I'll have a big suitcase and a rolling, more compact suitcase. I'd like to avoid taking the big one with me to London on the 31st but am not sure what to do. Is there a commercial luggage storage facility at T5? I'm guessing from the replies above that I couldn't leave the big suitcase airside in the bowels of T5 overnight?