In this instance I would go in on a slightly different approach.
You have created a valuable working relationship with your client and have given him a set of standards over a period of time. Does this hold no value to him, does he need reminding of this.
Will the person that he has had the quote from offer an equal and or better standard of end product. If he can get an equal standard £70.00 cheaper then that is good business sense.
Is he pulling a fast one and trying to make you sweat - as you have said.
Has he become dissatisfied with your service and has taken ths opportunity to change service provider.
If you feel that you have given him a consistently high end product that he can rely on then I would pop in and tell him how you feel and what he might be losing.
You should never have all your eggs in one basket!!!!!!!!
We have over 500 residential customers and lots of smaller commercial clients, we also look after all the window cleaning to the head office of a hgih street building society and look after the refurb cleaning (external) for one of the top 3 supermarkets.
I am selling our residential & smaller commercial work to one of the lads who has worked with us for two years, I am making a handsome profit that we can put back into other areas.
This will be a time of vulnerability but it will only be so for a few months until we establish other avenues and other working relationships.
It is easy to become complacent with where you are at and it might take something like this to get you out there and finding the next stage of your business.
However you handle the loss of this work make sure that you use it to never be in this position again
Good luck
Rob