"...OK, I've checked out the cost of getting someone to rotivate it. It's
only about 150 square yards but I'm getting quotes of between £250 to £450,
which seems excessive....."

I agree.......You could get 2 men with spades to dig it by hand for less.

"....One interesting thing which emerged is that there is something called
"Breakthrough" or "Breakout" which apparently you mix with water and spray
on the ground 24 hours before rotavating ......."

Never heard of it.
Try digging a spade full of soil and look at how dry it is through the spit
depth,
If its really dry then water WELL 2 or 3 days before you plan to work(That
should ensure Rain on the day you are going to rotavate)

Rotavating will give you a hard "Pan at the bottom of the area you work, and
the wetter the soil the worse it gets, caused by the blades rubbing along at
the bottom of their cutting depth.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk