Oh yes, I also have great memories of Coombe's boating lake and play area - my wife remembers them fondly, too - mostly from the 1970s. I remember getting a sore head when I didn't duck enough before running through those concrete tunnels! Also recall huge blisters on the palms of my hands (stop it!) after a couple of hours rowing on the boating lake - I'd not realised that wet wooden oars would cause such things!
I thought the cafe was much better then, probably because it was part of the grand old monastic building, and those slides with the enclosed top parts seemed much higher as a child, too. I remember being fascinated also by the talking Mynah bird in the aviary near the boating lake - the first time I'd ever heard anything non-human talk!
And do you remember the pub in a "hut" type building near the path between the monastery and the lake?