Discovery TD5 gearbox issues

My gearbox is doing the old thing of sticking in gear till it warms up and when it does change gear its a bit clompey, i have had this issue before and changed the filter and oil and normally this sorts it out over time but dont think it is doing it this time. I am starting to think there maybe a problem with the gearbox as it has done 180k and may need some attention! Anyone know of someone or a company that deals with these gearboxes and knows what their doing without taking the pee in the south Essex ish area i would be grateful, thanks

I would try changeing the oil first with some oil flush in it and then changing it again after a couple of hundred miles and see what it's like after that, could be cheaper than pulling the gearbox out for someone to tell you it's ok.