This is the Thorn EMI Heating Marathon 44/54 there are two models B & C - each has a heat output of 12.9 to 15.8kW and they're both very inefficient by today's standards at a miserable 65% - a modern boiler will be 90% efficient as a minimum.
They ceased production in 1983
As for manufacturer I have seen trade discussions that claim Worcester boilers to be reliable and easy to install.
As for type of boiler the choice is Combi or not - all boilers are condensing and room-sealed which means their exhaust is sent outside the property.
A combination (combi) boiler is better for a small property - it gives hot water only on demand and therefore has lower running costs - the downside is greater complexity and a higher risk of failure - the potential for a slow rate of flow particularly in cold weather - and no airing cupboard hot water tank - no immersion heater backup.
I think the new regulations call for a 22mm gas supply pipe - most older installation have a 15 mm gas pipe.
https://www.worcester-bosch.co.uk/homeowner/boilers
Good luck..., July 2015
