Following advice from others here I did my thermostat at Easter. If you don't, in cold weather the engine never gets properly to temperature (~70 degrees max depending on how hard you're pushing) and will be using much richer fuel maps among other things. Having done it once I could do it in around 15-20 minutes now. You need to remove the engine sound cover and boost pipe to gain space. The worst problem is getting to the screws holding the thermostat housing into the block next to the alternator. Long ball-headed allen keys (Lidl) and a curved 10mm ratchet spanner with a 1/4" socket adaptor were required tools to get into the space available. I couldn't get the coolant pipe off the plastic housing that goes over the thermostat as the jubilee clip holding it in place was in the wrong orientation, so I just undid the housing and released it, breaking off thermostat retention fingers in doing so. The thermostat was held in the block by the big o-ring ; once the o-ring was dislodged, the thermostat came out readily along with a couple of pints of coolant. I tried using the workshop manual method of pre-locating the new stat and o-ring in the pipe housing, but couldn't get it to sit into the block properly with the hose still being attached, so instead put the new stat and o-ring into the block, then fitted the housing onto it, re-installed the screws by fingertips in the space available until I could get tools in. Finally, topped the coolant circuit up, bled at the radiator nipple and then the expansion tank lid. Replaced the coolant sensor 3 months later, which eventually solved the temperature issues I suffered all last winter (both a knackered stat and a dodgy sensor).
Any half-decent ODB reader will do what you need - I use an Xtool VAG 405 which I found conveniently also does throttle body adaptations, but I used it a lot for reading temperatures (engine sensor and dashboard sensor) this spring.
