Ok, I can personally highly recommend SGS Engineering Colin (other suppliers also available

). I used these to replace both struts on my old C5 where the tailgate was knocking me out after walking into it all the time expecting it to lift! A huge improvement.
Note: Always replace in pairs - when one goes the other is sure to follow and not replacing both at the same time will put additional strain on the new one.
Your part number is: 8731J8 and SGS have an exact part here:
https://www.sgs-engineering.com/gsc1197 ... gate-strut
Their price is only £15.59 each inc. VAT and includes a 2 year warranty and the new end fixings. £6.99 for next-day delivery unless order is over £50 - but still a huge saving - and they are British made!
Citroen will charge £52.94 each so a no-brainer.