Ronseal is a retail product rather than a Trade rated product though having said that it will provide excellent performance for your requirements, Wickes also sell a clear wood preserver sold under the Wickes name at around £12:00 (?) for 2.5Ltr which is probably comparable with the Ronseal.
Beginning to worry now about having it up there on asphalt at all, and noting that the floor drainage already doesn't seem good. Mr. HandyAndy - Really
Ah, yes. I was concerned about the water pooling as well. It is a purpose built outdoor patio so no issues with regards to the weight. I did have a couple of roofing people take a look and they said a flat roof can never be completely clear of water and the best they could do was put down a rubber membrane on top of the asphalt but that wouldn't resolve the water pooling issue. So there doesn't seem like anything can be done about it.
It looks like you might have cleared a whole load of slabs? Which may well have been an integral part of the roof design, rather than a patio; even if it appeared as such.