I'm afraid I've only just seen this. (I do wish AB didn't have so many categories.)
Anyway, much the same as the others have said. First, ask Building Control if there is any record of their involvement in the construction of the conservatory. If so, then records should show Approval for the foundations.
If not, then, to answer your question, Anyone can dig the hole for you. It's a bit of a faff though. You'll need an excavation at one point on the perimeter. Where the conservatory joins the building would be best. You can then see the depth and its relation to the building's foundations.
If that's a problem, then pick a spot halfway along one side ....... and dig.
You'll need something like 600mm wide (2'), and around sufficient perpendicular distance from the conservatory to actually get in there to dig.
Gardens are usually pretty easy going. One chap should be able to do it in a couple of hours.
Grab a passing Young Farmer, or anyone beefy enough for a bit of digging. :o)