Pay as you go sim?

Digimobil do a pre-paid SIM for varying amounts, starting for as little as 5€ if I recall

Thanks Dario, I've looked at digi and it's still a monthly payment with no carry over.  

I'm hoping I won't have to use it so looking for the "old fashioned" top up when it runs out but may have to go with it

I’m not aware of any of the providers that offer that - most seem to want a monthly payment.

At least with Digi, unused data carries over to next month

Thanks again 

Get someone to send you a UK a pay as you go, and then if you need to top it up do it on line. I did this for years with Vodaphone.

Thanks, I'll try that

Except that Vodaphone UK have introduced roaming charges when used outside of UK

O2 don't charge roaming fees...(yet)

If you do go the UK Sim route, couple of things to watch out for

1. Roaming charges - make sure you go with a provider who doesn’t levy roaming charges
2. What network will the provider you’ve chosen roam on in Spain?  No point getting a SIM that has no roaming partners, or roams on one of the networks here with patchy coverage
3. What’s the purpose of the SIM? If you need it for emergencies only, probably fine - but if you are needing it to receive SMSs / Messages from any organisation in Spain, be aware that some organizations don’t accept a non-Spanish number.