My Humax remote 'OK' button was not working 8 times out of 10.
Remove the batteries before starting.
I used a very small kitchen knife to open my remote control, just easing it around the middle section until one of the lugs popped open. Then just move the knife around carefully, prising the front from the back until each lug pops open. Take extra care at the ends as these are the tightest fit (one lug snapped on mine, but the case is so tight when closed it didn't make any difference when snapped shut)
Once open, carefully remove the circuit board (mine had 2 small lugs holding it near the battery end which can be carefully prised apart)
I cleaned the rubber pad with some 'Sticky Stuff Remover' (available from Betterware and others) on a clean kitchen towel. You will find it all clips back into place very easily, and it now functions perfectly!