LOL and embarassing and yes --STUPID.... that's me!!
After puzzling about all this and finding that there is no obvious spare part or Rimmers Part No. for the clips (the air filter housing is sold with all 4 fasteners attached..)
And then a helpful discussion with MGF&MGTFBITZ; I then (suddenly) realised the following (yes, simple and 99% obvious)...
The two fasteners at the NEARSIDE position of the air filter housing are attached to the UPPER casing of the housing, whilst the two fasteners at the cylinder block, OFFSIDE position of the vehicle are attached to the LOWER part of the housing. OK so far?
Thus the two fasteners at the Nearside position are re-affixed by pushing DOWNWARDS on the buckle of the clip, whilst the two clips on the offside position are re-affixed by pulling UPWARDS on the buckle part of the fastener. SIMPLES.
Yes, unless like I was trying to do, which was (vainly) attempting to re-affix the nearside position clips by pulling UPWARDS9!!! (like the offside clips). Not so!
Hope this makes you smile or shake your head in disbelief.
Regards
Ian