I've seen a few people asking about thiis. It's hard to find on their website because it's under "Family Facilities", but this is what it says:
"Lockers are available on the lower ground floor. A £1 coin is required, but this is refunded. Lockers are large enough for large backpacks."
The only other option is the Backpackers Hostel between the bus and train stations, which was over £4 last time I used it.