Update..

The problem is now fixed.

I took it to what I can only describe as the worst garage in the known
universe, the garage who where trying to sting me for a new  ECU, so I
recovered the vehicle.

I drove back to Bournemouth at 02:30 in the morning to avoid the
traffic, foolishly taking the toll road thinking it would be a nice
straight run only to find one pay booth open and a big queue of traffic
all beeping there horns at somebody who had forgoton there money.

I got a local chap I know (auto electrician of 25yrs) to take a look and
he traced the fault to the Bitron device under the front left wing.
Upon further investigation the unit had beed previously levered open and
   messed with then gaffer taped up.
He charged me the £30 callout fee, not bad really.

He said the unit wasn't doing what it should be, so I opened it up again
and found a broken capaciter on the board.
I replaced the capacitor for 11pence from Maplin 100uf - 50v
£4.99 for a roll of self amalgamating tape to seal the unit back up.
Car fans now work fine again.

Car electrician blokey tells me there are three different types of
cooling systems on 406's - Early one's have a thermostatic switch that
turns the fans on, later Mk1's have the Bitron device like mine, where
the thermostat is just a temperature sender, not a switch.
Later HDI's the cooling system control is part of the ECU, is this correct?

Thanks for all your help

Andy D