Glowworm 38 CXI combi hot water disappering and coming back?

I have a 6 year old Glowworm 38 CXI combi boiler which delivering hot water only for the first minute and after that the water gets cold. However, the hot water comes back after a minute 1 or so. It has always came back so far. If you turn off the water when it is cold, normally the hot water is coming back sooner.
The problem came after an engineer filled my central heating system with corrosion inhibitor. We asked him to do that because we put a nail through a central heating pipe and having refilled the heating system with normal water after the leakage had being fixed. The system was going without or very little inhibitor for 4 month.
I have talked to 2 engineers and both of them thought it is likely to be the heat exchanger. The first one recommended us to contact Glowworm and they quoted a repair inclusive some spares for £280(expensive). The second engineer said it is probably not worth to repair the boiler because it is not a good boiler and the fault it is likely to come back again if has started + plus that the repair is expensive. The boiler is not perfect for the house either, a house with a loft extension and 3 bathrooms.
What to do? Find another engineer, go for Glowworm repair service or just leave it as long as possible or going for boiler repair plan with some company and wait until the initial period is covered. The system is working anyway.