You seem to indicate that you may be trying to start it on the "old
fuel" that was in it when you got it. Forget about the "leaded v
unleaded" argument" for a minute. Gasoline older that 60 days will be
much less volitile than fresh petrol and often will not ignite except
in autos with a strong electric starter. Replace the old fuel now.
Drain out the carburator completely. Ensure that new fuel (leaded or
not) is getting in to the carburator. Pull it over hard once or twice
with the choke full out and the throttle half open. If it starts
-fine-if not -choke half out anf try again and a couple or tries with
no choke at all. If that doesn't work the carb jets may be plugged
with old fuel residue (gum) and must be removed and cleaned. There is a
spray product that may help you do this without taking it to a shop.
Lead used to be added to petrol to act as a cushion for the valves so
that their closing impact did not damage the valve seat and that is
still important on older engines. Leaded or unleaded has nothing to do
with the combustionability of the petrol and wont effect the engine
starting operation. If it starts--I would get on with the
rotavating--the inherent valve seat dammage may never occur--or you can
use the additive from Halfords for it.