Hi Pete
Assuming that this is happening in Great Britain.....
My view is that it makes no difference whether the MHO Regs apply or not.
Whoever gets this worker to do the digging has various duties towards them, mostly via Section 2 of the Health and Safety at Work etc Act 1974 if the worker is employed, or Section 3 and/or 4 of HSWA if they are self-employed + an explicit duty under the Management of Health and Safety at Work Regulations 1999 to do a "suitable and sufficient" risk assessment.
Then, of course, the worker has duties under Section 7 of HSWA if employed or Section 3 if self-employed.
HSE statistics such as they are demonstrate that many workers lugging materials and/or equipment around sustain injury from such work.
I have always been of the view that the UK via HSE (and its smaller sibling HSENI) made a mistake in its approach to the "transposition" of European health and safety Directives by routinely doing this by making new Regulations, almost invariably under HSWA (with the notable exception being the Working Time Regulations).
The UK could have turned round to the EC and could have said:
We will tranpose this Directive by using our existing law, HSWA backed up with guidance and perhaps giving some of that guidance the status of an Approved Code of Practice.
There would have had to be a few exceptions to that rule in that if using a Directive as an opportunity to consolidate previous UK legislation, any new Regulations had to meet the test set in Section 1(2) of HSWA which is the bit that says that any replacement legislation has to maintain or improve on standards achieved by what came before.
....and so we might have needed e.g. a mini PUWER requiring dangerous machinery to be guarded so far as PRACTICABLE (NOT reasonably practicable) except with some rules diluting that a bit as done in PUWER as made.
However, that is nowt to do with transposing the Directive but rather sticking to the rules set by Section 1(2) of HSWA.
When I joined HSE I got a personal copy of the 1976 edition of "Redgrave's Health and Safety in Factories" often described as the "Factory Inspector's Bible".
It was over 2000 pages long and it didn't even include the Offices, Shops and Railway Premises Act 1963, even though HSE Inspectors were responsible for enforcing OSRPA in many premises.
Then in my next job I got a copy of the 2000 edition of what was now called "Redgrave's Health and Safety at Work" - some consolidation of legislation for its own sake but mostly done at a time when it was convenient to be done at the same time as transposing EC health and safety Directives.
This 2000 Bible was still over 2000 pages but the pages were larger, but the font size for the case law was in smaller text so making it more difficult to read.
What had essentially happened was the the legal standards hadn't changed much but the Bible had got bigger. I doubt that this would make life easier for anyone who needed to assimilate legislative changes, where the only significant one was the explicit duty to do risk assessments - clearly previously implicitly required if someone was to show that they had done what was e.g. reasonably practicable.
HSE had fallen into the trap of just drafting legislation to transpose Directives AND consolidate existing legislation as a habit, with the result being MORE legislation to no visible purpose.
Now if in the scenario you describe we brush aside MHO entirely you still have to assess AND control the risks and you can't ignore those risks associated with heavy manual work, but if you don't just jump in with a "Manual Handling Risk Assessment" then perhaps you are less likely to give insufficient attention to all the other risks of the task, whether that be striking the underground services, falling alongside or into the hole, exposure to UV radiation, ground contamination, Giant Hogweed, or whatever.