"Not sure that Combi Boilers are very well thought of, researching the internet"
Combi boilers have been in widespread use, here and in mainland Europe for many years, and I'm certainly not aware that they aren't well thought of. They are very efficient and reliable, and in normal use require only routine maintenance throughout their operating life.
I would not recommend spending another £450 on yours, it has given you 12 years of good service, and it's time to buy a new one.
This time, go for a condensing combi. My personal choice would be a Vaillant Eco TEC. All boilers in this range are SEDBUK 'A' rated.
I have a two year old system which is a condensing combo boiler. It is tested each year and runs at 98.9% efficiency. It's a Worchester Bosch Greenstar which was sourced for me by a friend who works in the trade and recommended it.
It is so cheap to run, I thought that the gas meter was wrong. According to my friend, it's best to go for a boiler which is capable of heating a slightly larger house than you live in.
I must admit, it's a huge improvement over the Economy 7 / immersion heater system I had before, but at the time there was no gas piped to the close so I had no choice.
I'm sure the FE's recommendations are equally good too. Spending money on a boiler which is wasting a large percentage of the fuel it consumes is really a false economy!
Last time I bought a boiler I was advised to get a Viessmann, about £1,000 cheaper than the equivalent Bosch Worcester boiler.
The only thing that went wrong in the first year was the shunt that switches between hot water & heating, and when I reported it to the guy who fitted it he contacted Viessmann straight away and they had a guy round that afternoon with a replacement part of higher quality.
With a Combo boiler, Size of Property and how its layed out Plus how big your Family all count on using Buying and one. Mine is a Wall hung condensing Boiler as it at least gives me a tank of Hot water if anything goes wrong