Off Peak switch doesnt work!!!

Hi Mike,
I been living in my flat for 3 years and I have always have used both switches, however I only seem to have hot water in the morning and in the evening (which is fine for 1 person but ttricky when i get visitors). I contacted Gledhill and I was advised that only the off switch should be used, hopwever when I do this the whole systme shuts down and the boost button does not work.
Please advise if this is a big job and would require the the year cover (very pricey) or I could fix it with one call out and part.
I live in London SW16 area, please advise of your cost to coming to see me, I been looking through the internet and you see to be the only one that knows about it.
I hope you can hlep me.Thank you for your time.
Odette