Central Heating – Thermal Store leak?

Gsea

System boiler in the garage (28 years old), in the airing cupboard is a Gledhill Torrent Thermal Store (15 years old) with a small F&E tank above it. (Access to the F&E tank is limited with ~6 inches between the ceiling and tank).

I think our thermal store may have an internal leak. Last week the F&E tank overflow was running so I immediately thought the ballcock valve was leaking and went ahead and replaced it, a few days later the overflow was running again. So, for the last few days with the help a camera set up I have noticed that the water level is increasing when heating the domestic water and when the central heating is on. There is no water coming from the expansion pipe or vent pipe that feed into the top of the F&E tank so it must be back flowing up the feed pipe. Am I correct in thinking that it’s probably one of the coils inside the store is leaking (pin hole). I think there must be two coils, a lower one which I think is the boiler flow/return which heats the water in the store, fed via the 3 port valve, and an upper coil which is the DW flow in/out of the cylinder connected via a thermostatic valve. Or, could there be another reason?

To compound my problem since changing the ballcock I now have a small leak coming through the tank where the ballcock is fitted, probably a result of the limited access preventing me getting seeing/accessing inside the tank to getting a pair of grips on the back nut to tighten sufficiently (ceiling and expansion pipe interfere with access). If the overflow exit was lower than the supply this wouldn’t be an issue, however when this tank was fitted, they have put the overflow hole higher than the supply hole, surely this is not right? Currently I’m manually draining the F&E tank when required.

 If a replacement store is needed, I notice they are quite an expensive Item, I’m also aware of the age of our boiler, so I was wonder if converting to a Combi is the way to go. I do like the idea of a combi and getting rid of a water storage tank in the house but a little nervous about performance.

Small 4 bed house, 1 bathroom and an en-suite shower, in reality, only the en-suite shower is ever used. The bath is never used and may eventually replace it with a walk-in shower, the bathroom is above the garage and the en-suite is at the opposite side of the house, mains water pressure is good.

Should I be weary of fitting a combi, or are modern day combi’s suitable for a 4 bed house? 

Cheers.