South West support

If the boost lights won't come on at all, I'd start by checking the electrical supply before assuming it's a cylinder fault. It could be something as simple as a tripped breaker, blown fused spur or a faulty boost controller. If the electrics are OK, then a failed immersion heater or thermostat would be the next likely causes. As you're only getting tepid water, it's worth getting a G3-qualified engineer to diagnose it, particularly if it's an unvented cylinder. They can safely test the electrical components and any safety controls. If you're looking for someone in Somerset/South West, a few companies that work on unvented cylinders are: NORTH SOMERSET HEATING (Clevedon) M.S.M Plumbing & Heating (Chard) R W Thomas Plumbing & Heating (Frome) Hydro Heat Limited (Somerton)