OK, Thanks for letting us know.
Your solution is fine for the moment, but we have to assume that the problem will recur for you at year end, or the next time a software upgrade is compulsory rather than optional, so better to find out what is wrong sooner rather than later. At the moment we have no potential solution for you and no-one else has reported anything remotely similar.
Do you have access to another PC? It would be interesting if you could install 1.4.8 on that and try restoring one of your backups onto it. Also it would be interesting if you sent us one of your backups as an email attachment to support@12pay.co.uk
EDIT: Just thought of another (perhaps more likely) explanation, which is that your download of 1.4.8 is slightly corrupt and the template file master123pay.pay in programfiles/12Pay payroll was corrupt and couldn't be opened.