Hi Neil,
We stay at Seaside every year & can't recommend it enough, quiet but not too quiet, near the sea, a couple of local shops just outside the complex & only a 10 min slow stroll to the front. 3 pools, night security, lovely grounds staff who are very helpful. Never had reason to complain. We have only ever booked with Joy Lettings on 1 occassion, because the villa we normally use was already booked. We felt the same way as far as 50% deposit was concerned & emailed our concerns and managed to negotiate a lower deposit. We see their rep every year coming & going to check on clients etc so that also seems to have improved because we were kept waiting for them on our day of departure to return the breakage deposit & take keys etc,which was a bit of a concern as we had a taxi waiting & the rep then was quite hard to track down. You may want to try Owners Direct too as their prices seem to be more competitive & you have the opportunity to speak to and deal with the owners of the property. Hope it helps