Is a supposidly reliable old free standing boiler worth just keeping?

If you have the money to replace the boiler I would recommend to replace with a combi. Much more efficient, will save you money on your gas bill and your hot water pressure will also increase. Also making more space in your cylinder cupboard once the cylinder is removed. If used as an airing cupboard to dry towels you could also fit a small radiator into the cupboard to get the towels dry. Hope this helps, luke @D&E Heat

You need to base your decision on factual information. I suggest you get this boiler serviced by a professional engineer, get them to carry out a combustion analysis that will tell you how the boiler is performing both aspects will be covered safety and efficiency. New condensing boilers are extremely efficient and reliable and some come with an excellent warranty as high as 10 years.

Be aware of vented system (hot water and central heating) replacement to sealed could cause leaks. Run new pipework when you go for 'pressure' in addition to new boiler.