Just came across this app on another forum - it's supposed to help reduce data usage. Has anyone any experience of it - and does it help?

Yes it is helpful, used it throughout January and I used 1.41GB and saved 560MB of data.

It remains on whilst connected to wifi though, so I have to go into the iPhone settings and turn the VPN "connect on demand" setting off, otherwise it seems to compress data over wifi too and messes with the stats (even though it claims it doesn't).

I also recommend downloading 'Onavo Count' which gives a clear breakdown of how much, where and when data is being used. Both apps work together well.