Hi there. In addition to what Nigel says, i can tell you where to buy an idealnpump for your situation. Vinaixa, in Mora d'ebre, where most people in my area have solved many issues on the water and electrical fronts. The pump you want is a BLOCH 300watt 230v ac pump, with controller. The price is approximately €180 for both items, the flow rate is more than enough for a gas water heater,  I have a cointra cob14 heater which allows a tap to be open whilst someone is showering.
Back to the pump, my current one is eight years old, previous one packed up simply because of the excessively hard water i then used. This one is stripped and cleaned annually, with a slight smear of silicon grease where needed.
Vinaixa is easy to find as it's on a corner on the main drag, free car parking 70metres away, oh and an English supermarket 100 metres away, just across street. It's called Mr.Cheap, lots of stuff there including frozen pies, pasties, bacon, sausages, gammon steaks etc, etc. Take a coolbox!! One final thing, if possible have your water storage slightly higher than the pump, just to ensure water is in pipes at all times, my water storage (15 ibc's) is 10" higher than pump. That's the bottom of the ibc's is above pump, not the top which would need some mega supports.