Success. The gentleman who answered the phone at Houdini was great. He knew everything and knows about the Parker 275. Key points:
Call Houdini on 01621 773590 and place your order over the phone.
The strut is sold as one complete piece. Ordering the plastic sleeve alone is not possible.
The strut model number, quantity one, is XO55054. Cost is £30.78 plus postage.
Just for reference, the hatch model is Lowline 555050M50. To encourage you to avoid falling on your hatch in future, remind yourself that a whole new hatch costs £441.