what boiler would i replace my thorn 44/54 with?

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