Hi Iain

Here are a few scraps which might help:

In the British Army WW1 Service Records on Ancestry, there's a Herbert Kermeen (unmarried) of Kirby Cottage, Braddan, age 20 in 1915, motor driver.

In the burial index, there's a Harold Sayle Kermeen (born circa 1921) who may have been a son of the person your Mum knew as Uncle Umby who was a bus driver in 1941.

In Ancestry trees, there's a very small tree which has a Harold Kermeen married to a Jane Sayle (no dates, but probably in the right time frame for your search), but the christian name of Harold may have been a guess on the part of the tree owner, based on the name of Mr Kermeen's son.

If you write a message to the tree owner and send it via the Ancestry message service, the tree owner might be able to help you.

Jean C