Anthea: Are your 6x great grandparents Joonathan and Rose Dumbleton? Either way, your 6x greats probably came from Oxfordshire.
I come from the Warwickshire spread of Dumbletons, who 'ended up' in Coventry via Tadmarton, Oxon, then Ratley, and then Baginton. My grandad remembers his great grandma Emma Everitt neé Dumbleton, born June 1844, who was 17 when her grandma Hannah Dumbleton neé Matthews died in 1861, aged 92. Incredible.
As Anthea says, the Dumbletons seem to have come from Moreton in the Marsh/Stow on the Wold area, which is near the village of Dumbleton in Glos. The first recorded Dumbleton was Bernard de Dumbleton in the 13th century, and Dumbleton means Dumel's hill. Somewhere I heard the name may have come from Belgium, but I'm not sure how credible that is.