I am not in the UK but I have a couple of suggestions. With those details it is difficult to say exactly what can be done, but here goes anyway .......
Most screen tops will do the job as long as it can be clipped onto the tank rim. (90% can be, but make sure), I am sure a local pet store would have these, if not they should be able to order one in for you if you don't want to order one on line.
Now the hard question, IF you have an under tank heater (UTH) you could fix your probe directly on this under the tank, just adjust your thermostat to your desired "internal temp" which you will have to take readings of with a separate thermometer which you probably have already.
There are two solutions I can think of now that may work if you use a lamp for heat,
1) Fix the probe to the outside of the tank as close to the "hot spot" as you can, move the lamp to a corner of the tank and adjust the thermostat to your desired internal temp. This will only work if the probe is close to the hot spot and you don't have the tank in a drafty area.
2) Snip the wire top just big enough for the probe cable to fit through and place it inside the tank where desired. To protect this cable from the wire a simple solution is to wrap electrical tape around the temp probe where it will rest in the screen "hole". The thermostat probe sensor will be larger than the cable diameter so you will have a gap (small as it ill be). By wrapping electrical tape at that point will secure the cable and fill the tiny gap. The only "problem" with this method is you will have the cable to contend with every time you open the lid.
Personally I use UTH's with the thermostat set at 86F right on the mat under the viv (I only have 7 set up) and a low powered heat lamp above for light and added thermal flow to exchange air in the viv.
I hope this helps some, the choice is ultimately yours to make, at worst someone may offer other ideas to contend with these, which won't hurt anyone.
