Thank you all for reading my post.
Chempat: I have not found anything on CFJ Smith - certainly no record of death, of course there's a chance he's still alive (born around 1922).
Rosie99: Unfortunately it is only Muriel's marriage to AC Cruttenden as shown on the 1939 register that gives a date. All others are hidden beneath redacted lines above and below Muriel's entry. The very kind people at FindMyPast managed to (I assume) take a peek below these redactions to tell me that there was another name 'Smith' against Muriel, which I how I found out she'd been married a fourth time!! No date was given.
I have been looking for marriages between Cruttenden and Howarth, as I have assumed, maybe wrongly, that Muriel was 'Mrs Cruttenden' when she married Mr Howarth, but of course there may be a fifth husband (who would have been husband number 3), in which case her name at marriage to Mr Howarth would not have been Cruttenden.
My search goes on .....