Had a fan belt changed recently by a garage .Ever since temp gauge moves down (but never up )from half way .( to quarter and then up again in a kind of 'wave' every minute or two . Any theories ?

Coolant has been in car at least four years / 60k and something is sticking ( thermostat/sender)?

Wire bumped and loose connection ?

Driving car ( after engine stopped for 2.5 hours and was cold ) ( or when belt broke and temp gauge went up to three quarters for about 45-60 seconds before I could safely stop ) without fan belt powering water pump has damaged head and engine is going to blow any second now ?Seems unlikely as it has has tons of mixed driving over last week or two .

Normal operation as these things are 'over cooled ' ? But why didnt it do it before?

Also they changed my oil , and car seems smokier,(due to colder running???) claimed they put in 5w/30 Castrol .Could there be a connection ?

Curious ...

It seems unlikely that you would have done serious, if any, damage to the engine if you only snapped a fan belt in the minute of driving that happened after it went. Usually you'd do damage if you lost all of your coolant and carried on. 3/4 isn't particularly concerning if it has a cause (fan belt) and only for a short period (as here).

As to you temp gauge issue, it could be electrical but I'd be more inclined to think it's sort of airlock situation. Do you hear sloshing noises from the dash board?

Definitely worth checking the electrical connections but I'd be more interested in making sure the cooling system has been bleed properly- on the 500 this was very easy but I'm not sure about the panda- someone else can probably advise.

Is the fan belt properly tensioned?

Changed coolant and ran clean water through system a few times .Panda MJ is so over cooled .Id say it would run fine with no water in reservoir/overflow tank .Even at minimum it never goes above half way on gauge .Anyway took car out for a blast up motorway and temp needle 'wave' may be going away .

Second the airlock possibility as the most likely cause. This sometimes cures itself with a bit of hot running.
The Multi's are definitely overcooled, as are most UK cars. Cars built for reliable running in very hot climates have the same cooling equipment as UK cars, so it's a given really.
On my Multijet I have the complete inlet grill blanked off all year round (except for the induction air intake) and nary any troubles. Have done this for years on every car I've had.

Yeah , there's nothing to be gained and 2-5mpg to be lost in a car that takes 5 or 6 miles to warm up fully .Even on my one hour commute with hardly any stops it seems to take best part of half an hour before she's giving peak mpg and power ( not that I'd be revving her hard until needle 'half way '.

Can be tricky getting balance right between coking her up with sub 2k rpm cruising and full throttle low gear accelerations ( which instantly take 1-2 mpg from consumption ) .Id swear I can 'hear' her get wheezy after too much slow driving .Cant contain with accurate pump measured 68mpg anyway .

Once had an Opel '89 Kadett GSi 16v ( like Vauxhall Astra GTE 16v but with no extras whatsoever not even alloys/ elec windows .Claimed 0-100kmh was 6.9 secs) ) .It had a digital dash .Temp gauge had 5-8 bar segments .On a particular cold ( not Arctic , just under freezing) Winters morning it didn't move AT ALL for about 40 minutes /28 miles ! .That car would do 40-42 mpg easily , in fact it was probably more economical than my 100hp .