Yes it is very good, although I haven't seen the current cast. Sit up front in the stalls if you can. As you say it is short / finishes early.
I had a look on the special offers sites I use but Six is not there I'm afraid, sorry. If you do decide to see other shows loads of them are on offer for "London Theatre Week" at the moment (need to book in the next day or two before the offer closes, it was extended already, but you can book for the next few weeks if the show is in the offer). Google London Theatre Week or just go to todaytix.