Find Ancestors

Right sorry for the long one but I'm at a dead end...any pointers? Basically my mum doesnt keep contact with her mother (Marlene Davies nee Edwards b. 15 Sept 1939 (has a sister called Joyce)). Her father Michael Wynn Davies (b. 26 April 1936, Wales, d. in August 1988) so I cant ask him either. On her mothers side Mum knew her Grandmother very well but she died in 1998 and was registered as Lillian Florence Edwards Crook (b. 17 Nov 1907). I can find info on Michael and Marlenes wedding and also their children (my mum and her brother) so from that point of view things are OK. However when I try to work backwards I'm stuck. So I thought I'd do a search on a marriage for Lillian (she was only married once) under that surname (Edwards Crook) but I've found nothing! Maybe I'm not looking in the right place. I also have no record of her maiden name so cant go on to look for birth certificates for her. On her fathers side Mums Grandparents died when she was very very young plus she spent most of her infant/early yrs living in Malaysia as her father was in the RAF. So am interested in ideas for figuring out their names. They were Welsh and probably originated from the Corwen/Wrexham area of North Wales. The only idea I have had is to get the birth certificates for both Mums parents (Michael and Marlene) as I have full names and birth dates for them they might detail both sets of grandparents. Anyone have any idea where I can get these without forking out £25 each?!