Malc, see under "Grant oilies" for information which will allow you to judge wilhelm's expertise.
You are right that all of the wall mounts are about the same for reliability, Heating World's Grandee (they seem to call all their models Grandee) and HRM use Sterling burners. These are really good as the pump is mounted away from the motor and so repairs are cheaper when the pump bearing leaks.
Warmflow and Boulter use Riello burners which are well developed and reliable.
Trianco used to use Sterling burners, but I think they now use Ecoflam. Thermecon also use Ecoflam, and though the burners are fine from a working point of view, they are comparative pigs to service, and I'm thinking of adding a surcharge to work on these.
Boulter particularly give excellent support, but then this probably applies to the other makes too.
Most of the wall mounts are available as internal or external except the HRM which is through the wall. This is quite an expensive operation to cut the hole. The outside servicing restricts work to dry weather, and it's a pain there's never a power socket to run a vacuum cleaner, so you still have to get in the house or garage.