In reply to John Lewis:
The extra pushing would normally be helpful, but lanes are wide and you need to move across the lane for the return paddle and sometimes wait for space if it's busier. If it's quiet (and it is getting quieter now we're a few weeks into term) it can really improve your times.
The other thing to avoid is the Aqua aerobic session which works like a wave machine (and comes accompanied with crap music).
I'd prefer to have the extra 5m, but 20m lengths are fine. It's a nice pool, usually quiet at lunchtimes, so I'm not complaining.