Hi,
If the front wheel is shaking, I would suggest this:
Lift the pram so that the handlebar is on the floor and the front wheel is in the air, this way you can see directly underneath the mono-fork (the fork which is attached to the front wheel). At the bottom of the mono-fork there is a very small screw (or hole if it has come out) this can be tightened with a very small allen key. If you have a hole there, then the screw can be purchased from
www.johnstonprams.co.uk
However, this only applies to Pro versions of the Powertwin, If yours is a non-pro, then again, I would suggest that you contact Johnston Prams to get them to give you a guide to adjusting the brakes on the front wheel which should stop the wobble fine.
(N.B. If your model is a Pro, it will say on the chassis 'Pro-Fix' or similar)
I hope this helps.