Hello Marion and welcome to Rootschat,
O.M. cemetery is a "working" cemetery and if you go during normal working hours, I'm sure one of the ground staff would be able to assist you. If it's like other North Lanarkshire Council cemeteries, there will be a record of all lairs kept on-site.
Perhaps going back Monday - Friday or Saturday before midday, you would find the chargehand, who , I'm sure, would be happy to help.