Total frustration

[Brian]

Try.... https://doubleogauge.com/intermediate-wheels/

OO Wheel B2B gauges sold by good model shops or one such is here... https://doubleogauge.com/doga-shop/ Their Intermediate is the correct one for UK OO

[Stese]

Brian has beaten me to the punch there... but simply put, it's the distance between the inside surface of the wheels. It is generally specified as 14.4mm wide.

if it's too wide, then you'll get riding up as the wheelset goes around a bend... being worse the tighter the bend.

if it's too narrow, then you'll get 'clattering' and riding up across point and check rails and the like

[Walkingthedog]

Don’t think there is much you can do with back to back on the Pecketts. The wheels and axles are minuscule.