In reply to girlymonkey:
Yeah, Voda customer service is awful. They have decent coverage, though - my work phone's with them and it's a handy backup, occasionally having reception when my Plusnet one doesn't.
I switched to Plusnet from Giffgaff as the deal was better - but Giffgaff tended to have awful coverage in remote areas anyway, and often said it had a data connection when it didn't.
If you're keeping your Giffgaff one, I think you have to use it occasionally (every 6 months?) to keep it live.