On Thu, 20 Nov 2008 20:48:57 +0000, Stephen wrote:

I have opened the box of electronics and given the relay a tap and it is
working for now but I expect it to stick again soon.

It may never stick again... but see below.

How can I prevent it from sticking in the future?

Replace it.
I notice there is a spark when the relays open. I'm not sure whether
that's the reverse emf in the coil or just the mains jumping the
contacts as they separate.

The contacts will be switching the motor current not the coil. Diodes
won't be much good on mains you need to fit a mains rated "snubber"
circuit across the contacts that are arcing. Something like:

http://cpc.farnell.com/FT00711

Possibly this is welding the relay stuck.

Quite likely so the simple tap above may not be a cure all.

I detect some rather basic questions, are you sure you are up to delving
into this box with live mains?

--
Cheers
Dave.