Many thanks for the useful replies.
Just to clarify, we took him around the garden for around 20 minutes explaining what we wanted doing.. We asked him to start in the back garden but all he did was the one side of the front garden. It is unfortunate we could not be at home but naively trusted him as he only lives around the corner and earns his living as a gardener!
We really didn't expect him to weed with a chainsaw!
The side of the garden he "weeded" is about 20 foot long and has been there for 30 odd years and is well established with bushes and shrubs. He has left the rest of it untouched apart from some minor trimming which we are happy with.
What we find most frustrating is that when we got him to come and explain why he had chooped everthing down in front of the greenhouse he just kept saying "it will grow back" .If he had said that he thought that was what we wanted or that the trees etc were diseased it would have been acceptable (maybe).
We threatened him with legal action and he looked visibly shaken and asked us what we wanted him to do to avoid legal action. We have told him we want the area reinstating with a mature tree etc. He is due to come back tomorrow with his proposal.
Unfortunately our neighbours have been away all weekend so we haven't been able to talk to them yet.