Hi pilot DAR
That copy just came off the interweb, was looking to revise speeds etc for a flight. Not flown for a while. Was surprised when I saw that statement.
I have just checked and the document is for a VH registered aircraft.
Now I have flown and have access to the legitimate POH for actual UK registered aircraft I can confirm the same restriction is in it!
Another aircraft in the fleet, POH has a revision of that page, revision 1 August 1972 where the restriction has gone, no mention of sideslips at all.