Roger Conroy wrote:

Does the thickness of the material used in antenna construction make any
difference to its performance? (Other than the obvious mechanical strength)
I have the option of using 3mm copper rod OR 15mm Copper pipe for 2m and
70cm 1/4 wave groundplanes.

73
Roger ZR3RC

Use the pipe, all other conditions being equal.

Whether hollow or solid, the larger the diameter, the
better off you are for that antenna. It is not a
thickness issue. A solid 15 mm vs a hollow 15mm diameter
offers no advantage, RF wise. But in a 15mm diameter versus
a 3mm diameter comparison, the 15mm wins every time.