coolant temp gauge not working 170 dsg

[1982robw]

hi there, we have a 57 reg 2.0tdi, 170 dsg, BMR engine code. we've had it about 18 months, it has full vw service history up to when we bought it. we had the dpf removed as its 110k miles and it was reprogrammed to eliminate the regen process last july. it has run fine up until sunday just gone.
my wife uses it mostly, it was only when I drove it I noticed the temp gauge was not moving from 'stone cold' position. it idles about 1100 rpm and feels sluggish when pulling away, auto gear change is a bit slower as it seems the car thinks its cold even when its been running for 10-15mins. I think the mpg is suffering because of this.
coolant level is ok, no previous leaks, no lights have come up on the dash, temp gauge doesn't move.

I have replaced the thermostat and coolant temp sensor which is located in the same pipe as thermostat with genuine vw parts, radiator is cold but top feed hose is hot, bottom rad hose is cool/cold.
if the water pump had failed I'm sure the temp would go sky high as I've had this happen on a golf mk4 1.8t.

any ideas?

thanks rob.