Jules - you beat me to it!
If you as a couple, book one extra 5kg, you can share it as long as no case is more than 23Kg.
as a single - (or if you get conned into booking an extra 5Kg each), it's wasted,
unless you take two cases, in which two Kg. minimum, per two cases, of that total weight is still wasted,
as that is what the additional case may weigh!!
You just can't win.
As Jim said - Thomson's don't really help there, because their website gives all the information on Baggage allowances,
http://www.thomson.co.uk/editorial/legal/baggage-allowance.htmlBUT - - - that doesn't help you on your individual flight's information.
That is one of the reasons, I bought a pair of medium sized Sub Zero G2 (Generation 2), suitcases,
described as the worlds lightest suitcases.
It equated to an extra 3+Kg between us compared to our previous pair of Tripp Lightweight cases, which were then formally,
the worlds lightest suitcases according to the makers several years ago!!