Hi to you from David, 'The Famous Forry'
This fix sounds simple enough. From your fault description I would think your filling solenoid has a worsening intermittent fault. Solenoids are priced about £25 and fittable by all good DIY persons. I will look up your part for you if you would like me to. Do you have a multi meter. This helps determine fault and is available for about £15 from all good DIY stores. It will prove very usefull throughout your life I'm sure. Awaiting your next comment regards David.