In my limited experience I'd be looking for another device in the house that is electrically noisy, but then you're not an electrician so not really your job... Good idea about using an inverter, I take it you are planning on running the boiler from the inverter for a while, few days perhaps? You'll either need a big battery bank or a way of charging the batteries. IIRC using a mains charger to charge the battery powing the inverter should decouple the supply enough... Anyway back to the inverter, reciently I've only used a victron inverter, expensive but great, everything I've run from it works perfectly....