Any vehicle A/C centre or VAG dealer or KwikFit or anyone else that shouts "we do A/C" will add in the dye, in fact some will be using R134 that has dye already in it, that's a point, Halfords might still small re-charge tins of R134 that come with dye in them - or the good old ebay if that is cheaper. Personally, I think that the dye just works once, after that the whole car gets traces of dye over it as the fixer will not bother to clean off any residual oil/dye! A proper fix is - recovery any remaining R134, pressure test using OFN, that should reveal any leaks as there will still be a small ammount of R134 and R134 contaminated oil left in the pipework and the OFN should force that out any leak sites, fix leak, repeat pressure test, release OFN, vac out down to ??milli bars and hold for ?? minutes, reload with the correct weight of R134, run system to check performance - ie pressures and resulting air temperature. So, will they do this - maybe, I'd think a quick bash in of some more R134 will be all that you will get, then if/when you need to return with same
problem they will try to sort it properly.
One leak site could be the condenser as it gets a lot of road debris thrown at it - look for traces of oil leaking out (wet), next one that I've seen on my daughter's
Ibiza 6K is on the filter/drier thing - its the aluminium can that sits vertically at the RHS front next to the radiator - there are two pipes going to it an"in" and an "out", the O-ring seals do fail at times - again look for "wet" deposits there.