Thanks for that, Tony.
The existing pipe locations (flow, return, gas) match the layout for the IG OV boiler perfectly, so will probably opt for the OV and not System.
Yes, I'm hoping my current Netaheat Profile will go on for 'ever'! I nearly changed it 2 years ago, when everyone seemed to be saying I was wasting hundreds of pounds a year. I didn't change it, and a few weeks ago decided to work out exactly how much I was wasting in gas annually by keeping the existing boiler.
Based on all the relevant criteria (annual kW used specifically on CH and HW taken from previous meter readings, current cost of gas per kW, current boiler stated efficiency, new boiler stated efficiency assumed at 90%) I calculated I was 'wasting' £60 per year on gas consumption by not changing boiler. This was a pleasant surprise, and I checked and re-checked all calculations just to be sure. Glad I didn't give in to all the official pressure to scrap the boiler without first doing the calculations.
So, for the time being, I have no supply and installation costs associated with a new boiler, no servicing charges, etc. Just the 'wasting' of £60 per year, which of course will increase if gas prices rise.
I must admit I have treated this boiler in a shamefully bad way! It has NEVER had any inhibitor added, since day 1. (The original installer said there was no point him adding it, since I was about to replace a couple of radiators after he left, and he left me to add it in myself. I never did.). Also, it has NEVER been serviced in 23 years. I did replace the fan some 10 years ago, when I took the opportunity of inspecting the combustion areas, and everything appeared really clean TO ME, even after starting to brush the flueways. Carbon build up was virtually nil. No hissing, banging, no cold spots on any of the radiators. The visible flame is always a healthy blue.
2 years ago the F&E tank was replaced with an expansion vessel, when we needed the loft space for a loft conversion. The system appeared to become even quieter after that, and no leaks in spite of the high pressure. Still never added inhibitor then, since further modifications with radiators etc were envisaged.
Now I feel bad for neglecting the boiler this way over all these years, so to make amends my next step is to flush with Sentinel X400 for 4 weeks, then replace 2 very old radiators, then flush again before adding X100 inhibitor.
I have developed a great respect for this boiler, and based on the above facts see no valid reason to replace it before it packs up, hopefully years hence.
I'm unlikely to get warning of failure, hence the need for advance planning. 3 cheers for cast iron heat exchangers!!