Jo,
Soak them over night in cold Chlorine water. A concentrated solution can
be found in any supermarket - usually in a flexible recharge type
plastic container. This is diluted according to the instructions - 1 of
conc. solution to 10 of water, if I remember well.

This will break up the propolis, wax and clear any other material - as
well as disinfecting the leather.

Rinse well next morning.

It will slightly bleach the pig skin - but will not in my experience
cause any problem to the gloves. Mine get the treatment at least once a
week in the season. At the end of which they are usually discarded.

Peter