Went into town to drop something off,the journey is 3 miles there,when I started the car it had 25 miles left,was in a rush but by time i'd gotten into town it was on 0 miles left.the engine wasn't cold either as I'd only just got back from somewhere.could this be a dodgy instrument cluster or the effects of having 1+ glow plugs blown?

No it's common the mileage left adjusts to how you drive you say you where in a rush so I take it you put your foot down the computer will be thinking oh this is going to be the norm on what is left in the tank that's oh far it can go if you know what I mean know when you start the car up next it might say you have ten mile range left according to your driving style .

Diddnt put my foot down,I rolled most of the way there and the cars in limp mode so I couldn't go fast if I wanted to

Have you got a scanner to see why it's limp mode the read out will still adjust for this any why if you clear the fault code and get it out of limp mode the milage will adjust again I can get down to zero come out to go and fill up and it says I,ve got 25 miles left you can have the light come on on the motorway say showing 50 miles left and if you adjust your speed you can go 20 plus miles before it says you've got 40 miles left . So I wouldn't worry about the gauge being out I,d sort why it's gone into limp mode then go from there .and the glow plugs are only for initial start up so don't have a bearing on fuel consumption .but not sure if that could be the cause of the limp mode as it would see a malfunction from start up .

Yeah it's come up as glow plug module circuit or something,I posted it on a thread last week if you wanna check as I'm on phone atm.it also came up as something to do with the instrument cluster.thats why I asked.limp modes been on for about a week now.before that it was intermittent and seemed to go off if I chucked round a bend or wobbled the car etc,so sounds like an earth or loose connection to me

Yer that sounds feasible have you checked for loose earths or any obvious signs of one being corroded read on here that some of the pins in the engine bay fuse box multi plug a can get corroded might be worth a look .

Checked the fuses but can't really tell if the bigger square fuses are ok as there's no visual way unlike the little fuses.any idea where the earths are that I could look at ?

Apart from the main one to the battery and the one to the gearbox I don't know where smaller ones would be but you will have some somewhere possible under the trim passenger and drivers footwell .

you would need to look at the engine bay and gearbox for the earth points, and ensure they are clean.