I had the same problem mate,take the six screws out the bottom of the backpack,open the backpack up then connect your pole hose,switch it on to try to establish where the leak is.There are three posibilities,
1 Leaking from the black plastic screw thread directly under the water container part
2 Hose clips that come with the backpack are not up to the job,replace with jubilee or O clips
3 The hose supplied with backpack is cheap rubbish,replace with minibore