George wrote:

a strip of copper 2 and a quarter inches long and half an inch wide
clamped into a suitable holder. As a guess I would say they would
probably blow at a bit over 50 amps. The thickness is, as far as I
can measure it on my caliper is 1/32nd of an inch

Are the holders enclosed? sounds like a sizeable amount of copper to
spray around if it goes pop ...

Does any clever chap know how I can calculate the current capacity
from these measurements and so adjust it by cutting the strip in
half, for example.

Anyone got a clue? Thanks George.

As a starter, this calculator

http://solderer.tv/wire-size-fuse-calculator

reckons you need 0.42 mm^2 of copper for a 50A fuse, which sounds small
to me, but would make your strip over 20x the required CSA.

Another site seems to give same ball-park sizes, so maybe it's not to small

http://www.powerstream.com/wire-fusing-currents.htm