Suitable alternative employment offer and maternity leave

[Neverknowingly]

Name changed for anonymity.

Just been told that my work location is to close. In the view of my company I have been offered suitable alternative employment at another site 50 miles away although they have indicated that they will pay statutory redundancy (but not their usual redundancy package) to anyone who does not want to transfer. The expected date of transfer to the new site is 1 August 2014 (following consultations etc).

Snag is, I am due to start maternity leave on 1 August.

I understand that I am entitled to a 4 week trial of the suitable alternative employment. Is that right - the role is essentially the same as my current (role although I have serious doubts as to whether that will still be the case by the time I return from mat leave)? This seemed to be news to HR although so do lots of things but I am not clear whether the trial period only applies when there is a different role and not merely a different location.

Can I insist that this 4 week trial be on my return from mat leave? HR are saying that I should do this voluntarily during July (ie before my maternity leave) but to me that is not going to be representative of how I will feel doing it potentially permanently and with an enlarged family but whilst not being heavily pregnant.

If anyone can point me to the actual legislation that would be very useful.