The first step is to approach the Mother and offer some reassurance that you do not wish to take sides in the divorce but would like to able to continue having regular contact with the children and offer mediation or family counseling as a way forward. Alternatively you can approach a mediator or counsellor yourselves and they will invite the Mother.
If your grandson has Parental Responsibilities (
http://www.direct.gov.uk/en/Parents/ParentsRig hts/DG_4002954 ) or in Scotland Parental Rights and Responsibilities it is each parent's decision who sees the child during 'their' contact time, so if your grandson has secured/will secure contact there is no reason why you can't see the child then.
Ultimately grandparents in England & Wales have the same rights to apply to court for a contact order, the only difference being the court must grant leave to apply first.