Efforts to obtain a replacement rim quickly locally thwarted (although Oakmere of Northwich were very helpful). I contacted Mogparts (Melvyn Rutter Ltd) by 'phone yesterday morning. Very efficient and obliging and the rim arrived this morning with replacement stainless steel screw (their part number ELA183 description "Headlamp rim 2/1995 on (Wipac"). Total cost £52.88. No doubt there are cheaper options but it will be fitted tomorrow and the problem solved (subject to checking the fixing screws every time i take the Plus Four out of the garage).
