Here you go Iain ....
Top box strapsBasically - drill out the pop rivets and use over size washers with a nut and bolt to clamp the straps.
May I also draw your attention to this set of Posts :
HOW TO: Check the most common faults & issues.
Spread across 3 pages, this is an alphabetical list compiled from reports within this Forum,
and on Page 2 under "P" for PANNIERS /PANNIER ( and Top Box ) RETAINING STRAPS,
and also under T for TOP BOX, you will find the above Link

Cheers,

p.s. - I haven't forgotten - Hoping to find some time this week to have another go !