Advice on remapping please

I know this has probably been well covered before, but I would be grateful for some advice per our Octy Mk1 1.9TDi 130 ('04)

I bought one of the fuel temp sensor 'mod kits' off Fleabay earlier this year and fitted it just before we went off with the caravan for a week at the end of March. I could not believe the difference.

Towing fuel consumption improved from about 30.5 to almost exactly 40, solo from around 51 to 68, and most noticably the torque; where I had to keep th revs around 2000 and never higher than fifth gear when towing before fitment, with the device it would tow comfortably in sixth at about 1600 (both equivalent to 60mph.)

On the way back in pouring rain Management succeeded in invoking traction control momentarily when starting off at a traffic island and at that instant the ECU warning light came on and would not go off. The ECU did not however change the map to the 'limp home' mode.

When we got back my tame Skoda man put his box on it and the fault showed as 'fuel temp sensor out of range.'

I have since found that the range is defined to 100C where the sensor value is 325 ohms minimum, but the 'mod kit' replaces the sensor with a fixed 180 ohm resistor so clearly fooling the ECU - although just why baffles me.

Two questions. Does anyone know the lowest value of sensor resistance that will fool the ECU but which will not cause it to alarm? Is there any other way of getting this sort of improvement without going to the cost of a full remap - I'd be prepared to go to £100 but £300++ is too much for me. I don't necessarily want the increased bhp, just the improved fuel consumption and torque, although I suppose one comes with the other?

I was talking to someone a while ago who has a Mk1 105bhp who's service place had (he said) plugged into the management interface and changed some settings which (he reckoned) gave hime around 115bhp. He said his mpg was improved and (especially) so was the torque, but search the Interweb as I may I cannot find anything about that.