gas safe will give you a list of RGI's in your area, other web sites will give you more options (although you really don't know who you are going to get), it really shouldn't matter what boiler you have it's just a boiler nothing really complicated about yours, it could be several things & i'm sure most of the RGI's on here would have a good idea where to start looking, however as per forum rules we aren't allowed to advise you on some of them, but have you checked page 11 (top right) of your link not only the boiler but the pipework ?
i hate to say it but as you are having trouble finding someone have you tried BG & their one off repair ?
i hate to say it but as you are having trouble finding someone have you tried BG & their one off repair ?