You have to be there before 4pm and show your white club membership card to qualify
A word of warning though, Ratho does get rather cold (as it is effectively a very large hole in the ground) so make sure to wrap up warm - just like if you were planning on climbing outside!!
However it does have a sauna, steam room and spa pool so remember your towel and swimming costume
For those of you without bikes (or cars) Lothian busses run a service, the X48, from Waverly via Haymarket, Costorphine and the airport park&ride to Ratho village. (rida card and day travel tickets are valid)
From the bus stop opposite the pub (as soon as the bus enters the village) cross back over the small humpback bridge and turn left onto the canal path. Walk for about 15min till you come to a set of new wooden steps on the bank to your right, asscent and enter
(a torch would be a good idea for the return walk)