Hi Colin,
You are quite right to assess your employee regarding the additional risks involved, I assume that you have in place some mechinism to monitor lone workers at various sites. I think that your advisors will be best placed to answer this question as it should be covered by your employment contract, apart from traditional maternity pay, there is no way the government is going to re-inburse you (exception to this is perhaps for her doctor to sign her off due to phsical risk/problems)
Most professionally drawn up contracts will contain the relevant clauses to enable you to switch her role within your business so that your not paying your employee for not working, perhaps light duties, office work etc, or a mutual agreement of a cut in workload/hours or to team them up with another worker to ensure that she no longer has to do the physically hard operations.
A couple of things to be aware of, a baby on the way, the current economic climate, might mean that money may well be tight for your employee, a calm, understanding conversation may well enable you to reach a compromise.
It is obvious from your post that do not wish to play the capability card, good on you for that, if your contracts have not taken this situation into account before now, get your HR people to pop the relevant clause in, so that when this situation arises again, you know your liabilies in the future.
Regards,
Rob