Not strictly DIY but I know someone here may have an answer.

My parents are having quotes to replace their 26 year old boiler replaced.
She's had two quotes, one using a Gloworm Flexicom 30SX and one with a
Keston C40. The Gloworm quote is significantly cheaper and it's obvious when
I look at the prices of the boilers on line. The Gloworms typically go for
about £700 and the Kestons £1600.

Considering the fact that the house is a semi-detached period house (no
double glazing etc.) that would happily house a family of five or even six
are both of these boilers suitable and is the Keston going to be a lot
better? Is its only advantage its increased power output or are Kestons
generally better anyway?

Thanks