Big Royal Mail - Special Delivery

I installed the Big Royal Mail module a while ago and have nothing but praise for it.

However I have a little issue.

When my customers select the special delivery next day service.

I think they are thinking that when they book this service on a Friday they think that their parcel will be delivered on the next day, Saturday.

Id like to change any wording that appears on the website as ....

Royal Mail specialdelivery® next day (Great Britain & Northern Ireland only : 0.484 kg(s))

to say next "working" day.

Can anyone point me to what files to change. I did find one place to change it but it didnt have any effect on my website wording.

A special delivery next day CAN be delivered by RM on a Saturday for £2 more.
Has there been a change to the module to allow an extra £2 to be collected if a customer wants this ?