The William who was born 1842 may have died and the name used again
Yes, it certainly points to that. Seanybear, have you looked for a death for this 1842 born William? Have you looked for a birth around 1849 for the William who appears in the censuses?
There is this marriage which is available to view on Scotlandspeople
1918 LIDDELL WILLIAM THYNE and STRATHERN ELIZABETH at ANDERSTON Glasgow
He'd be an old groom if it's the right William ... (he's 54 in the 1901 census

) However your rumour that William Thynne Liddell married an Elizabeth S seems true, but maybe the wrong generation?
