I found that the pipe was partially hidden by the boot lining, right in the corner. The two bits of pipe were just hanging down instead of being joined.
I think my Mk2 only has the pump on the washer bottle under the bonnet (and heated washer jets on the front

), but some have a second bottle and pump for the rear washer.
If you can hear the pump going, and there is water in the bottle, but you're not getting anything out the washer, you probably have a break in the pipe.