Have you asked them what they would be happy with?

In reply to Keith:

Thanks - We have but everything suggested seems to have a level of unfairness to it and now the unions have got involved and want a fair system putting in place so I was just wondering what other organisations do

In reply to Ruth :

All our overtime is voluntary is very much on business need. Teams have to request it then the supervisor and workshop manager assess the need and decide how much to authorise (if any). We do pay extra for Bank holidays and this follows the same process but in all honesty it's extremely rare that bank holiday overtime is needed or even approved. If the unions are now involved then ask them for their suggestions. Pulling names out of hat seems pretty fair to me, but if they think otherwise then i would be "engaging with them" to see if they can come up with anything better.

In reply to Ruth :

Well I think what ever you come up with needs their buy in so they need to be involved in its selection

One solution might be to adapt your random draw in that once you have been drawn out of the hat once you cant go back into the draw for the next X number of draws to give everyone else a chance.

Fundamentally I might also look lounger term at why I was paying such an enhanced premium for BH working when clearly lots of people are willing to do it but thats a whole different debate.

Put them on a rota.

Develop a rota

My take used to be run from Christmas as the start of the year. Those who volunteer to work over Christmas and New year get 1st refusal on Bank Holidays. As with any rota theres only so much you can allow a team to dictate their hours to the employer. It all depends on the sector and individual business needs.