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If you are looking for an occasional use PAYG tariff, PAYG 1 would ideal for your needs. Please see the link below.
The only thing you will need to do is to make sure you use the phone for an occasional chargeable connection during a 180 day period, to prevent disconnection for non use. This is something Vodafone clarify in the Terms and Conditions.
If you ask for PAYG 1 a store will know exactly what you mean. If they try and sell you a Big Value Bundle, you only want PAYG 1 for your occasional use. The information for PAYG is on the website on the link below.