We had this problem recently with our year or so old td. The first chappie who came out told me to reach inside and clean the back bit where water can affect the element. As the book doesn't say to do this, just the filter at the front and empty the water (we have a condenser one, btw) then we hadn't been doing this. It continued to trip so we called them out again and the second chappie replaced the heater which he said was gunked up and the problem hasn't reoccurred.
Try cleaning the filter very thoroughly and also give your washing machine a maintenance wash as the chappie said that due to washing machines using less water these days, there can be too much soap residue left on clothes. This produces matter when the clothes are tumble dried and the water tracks along this gunk and affects the element. You can buy maintence wash stuff for the washing machine on e-bay. For now, though, you could put the clothes on a rinse wash in the washing machine after you've done the maintenance wash and this might help take away any residue left on the clothes. Then try tumble drying them, making sure you put very heavy wet things like towels and bed linen through a separate spin cycle in the w/m first.
I've thought of something else. If the t/d is tripping the second you put it on then you probably need to wait for it (the element) to dry out. Empty it of any clothing, leave the door open for it to dry out and wait a few hours or overnight before you try again.
Please remember that I naff all about electrics and stuff and this is just what I remember from when we had the same problem a month or so ago.
Thanks that's fab, I really really don't want to buy a new one as they're so expensive, we need a condenser one because we have to keep it in a cupboard which is probably not good for it but it's either that or do without.
Update - We took it apart and cleaned stuff and sprayed WD40 on bits of it (DH's bright idea with me stood over him saying are you sure it's ok to spray it on that bit/ spray that much!) so then he switched it on and smoke started pouring out so we dragged it in the garden and left it until the next day then I tried again thinking as it didn't burst into flames or trip the electric even with the smoke then maybe it just needed to burn off the WD40 or something and then it would be fine...........no it tripped the electric after about 20 mins so we've given up and got a new one.
This did the trick, I had my drier up and running again in less than 10 minutes. I simply took the back panel off dried the lying water with a cloth then blow dried through the fan vents for a couple of minutes...