Hi alfie.
Im not sure k tetleys west bunks will fit your roof aperture, some one may be able to correct me on this.
I assume it is a factory fitted devon calthorpe roof you have (sorry difficult to tell 100% from the pictures),
in which case you would be best to look for some original 'early' devon bunks (later ones have a different fixing)
see http://www.campershop.co.uk/roof-parts.htm#bunks" onclick="window.open(this.href);return false;
If your not fussed about originality I think Just Kampers still sell an aftermarket type which would be easy to fit.