Type of boiler chosen
It may seem a very obvious point, but it’s so important not to ignore the obvious, don’t fit something that need to be changed later because of something you overlooked.
Combi boilers may be effective & well priced, but if you have high hot water demands, then you may struggle with them, so you should make sure the boiler can cope with current & future demands for heat & water.
Mains Gas boilers are very popular and priced competitively, but in the LPG or Oil fired market, you will need to do a lot of shopping around to get the best deals.
Get the highest energy efficiency rated boiler you can for your money.
Don’t hurry your decision, plan for the future, use existing infrastructure where you can & always get multiple written quotations from at least 3 or 4 professionals before coming to a decision – having a few quotes could also help you negotiate a bit on prices too!