I popped into Harrods today to see what they have.
The only soft case with 4 wheels that was light enough was a Delsey Fiber Lite at 2.9kg. The case itself was a little small (32 litres I think), which is the only reason why I haven't quite bought it yet, but it looked good enough.
If I can't find something a little larger, I think I'll go for this one.