Thanks for putting the record straight WHBM. We thought of it as a 748 replacement.
I don't remember considering using the M45H but I do remember, after the 146 had several years in service, someone in authority asking us to consider using two R-R Tays. We said, more or less "if that's what you want you can't start from here" and referred them to (by then) our Weybridge office where they had investigated a Tay-powered 1-11.