I'm looking for a backup SIM which will keep a phone going but not require
a monthly top up.
Ideally Vodaphone (to diversify networks) but retaining credit long term
is more important.
A standard Vodafone PAYG sim will keep its credit as long as you make a
chargeable call, or top it up, every 180 days. https://www.vodafone.co.uk/mobile/be...ou-go-sim#help
Under all the blather about bundles, if you scroll down you will find the
details here https://www.vodafone.co.uk/mobile/be...-as-you-go-sim