Patrick must have been smiling down on dulra today. the pump has decided to co-operate and is now running (or was when i left the farm this morning).
two things that helped were:
1. releasing the extra air i had added to the inner tube
2. starting with the delivery valve shut and opening it gradually over about 5 minutes.
i measured the volume being delivered - it's about 1.3 litres / minute, which is more than twice as much as the PVC pump when i measured it (0.5 litre/min). this isnt necesarily comparing like with like because the water levels in the tank might have been different, but great news either way.