Firstly, you mention that you have received a letter. Have you used this service before, or is this letter a marketing ploy? I would advise you have gas appliances serviced for obvious safety reasons. Don't take any chances. But you aren't compelled to go with any company. Do you have some family or friends who could help you out?

Now onto finding an alternative...

I've searched for that 0870 number using Google and it doesn't come up.

Warmsure appears to have a website at www.warmsure.co.uk

No contact details on that page.

A look up of the registration information for warmsure.co.uk web address (see here) gives the same address. The registrant's name is Chris Jessop. A check of BT Directory Enquiries reveals a Chris Jessop, also in a HU5 post code. Note that the address given is a PO Box, so it's not actually a physical address. I won't post the number here, but you can look it up if you want as I assume it's his home number. You could phone him to see if he can give you a clue where the 0870 number goes. Perhaps it goes to his place of work.

Which leads me on to the next piece in the puzzle. I wrang the 0870 number and it went to an answering service and said that the person at extension 6181 was unavailable. So this is obviously a DDI number in an office that has many extensions. If we can work out where he works we may be able to go straight to this extension. 

The only clue is that it is operated in partnership by the Eaga Group and Ideal Boilers. The contact page showing various offices for Eaga Group is here.

I tried the following numbers by taking the first few digits from some of their locations and putting 6181 on the end:

0191 2476181 gives a redirect message to another 0870
01858 546181 not recognised
028 8775 6181 not recognised
0191 350 6181 not recognised

Ideal Boilers proves more fruitful. The address on the website is the PO Box quoted in the OP. Most numbers on Ideal's contact page start 01482 49 and dialling 01482 496181 gets the Ideal Standard sales office.

A check of the registration information for warmsure.com (see here) gives the same address with Chris Jessop's name and contact number 01482 498607 and email address Chris_Jessop {@} caradonplumbing {dot} com

Dear Dave,

Thank you very much for your reply, I was quite bowled over by how much work you must have put in and am very grateful
The central heating was put in under a Government scheme for pensioners and belongs to the Government for 7 years, so there is a free annual service, and Warm Sure is a private firm which works for the Government, so I have to deal with them.  I have now got an appointment and once again, thank you very much.  Jacik

As seen on last nights Watchdog

Thank you Dave.  Only just started using SAYNO but was fortunate enough to come across your forum with the alternative number for Warmsure. (01482 498181 Managed to get through but told it was the wrong extention so they put me through to their service dept. hopefully I wasn't charged for an 0870 rate with the transfer. If this  so it will show on my next bill and I will let you know.  I did have to wait approx 10 mins for someone to answer but if at a cheaper rate it is a little more bearable. So thank you for all the work you put into finding this number.  I will now be trying to find out where/how to add my voice to stopping this sort of rip off.

I found this site this morning whilst looking for a contact number for Warmsure, I would just like to let everyone know that I have found a new contact number for them (by confusing their telephone system with the # key)

The number is:- [b]0800 4081463

To Welshwonder,
I too have had a letter from Warmsure about my first annual service which actually quoted the number you mention. It was followed by a call from Warmsure cancelling the date they said the engineer would call because he'd reported in ill and gone home. Well, that's what they said. A new date was negotiated which later I found I could not adhere to so I tried to get back to the company to change it. I found it impossible to get to speak to anyone on that 0800 408 1436 number and, being a pensioner, I am not ringing a chargeable number to be kept hanging on at an inflated cost. I will be sending a letter to the address shown at the top of the letter I received (keeping a copy of course) and then it's up to Warmsure to sort it out.
DancingMan81