I assume that the UK version of the GRO has records of all marriages and all divorces, so marriage cert from 1964 plus absence of divorce records would prove?
Do UK marriage certs have the pre-marriage marital status listed e.g. single, divorced, widowed like Irish one do? If that is the case, having "single" rather than divorced on the 1968 cert may invalid?
On issues like this, the one mistake people often make is confusing a church marriage with a civil marriage.
There are 3 types of "marriage" as follows:
1. Civil ceremony e.g. Registry Office - this is recognised in Law.
2. Combined civil & religious ceremony in church where civil paperwork is completed alongside the church paperwork - this is recognised in Law.
3. Religious marriage ceremony in church - this is NOT recognised in Law.
As well as checking for the GRO records, you need to confirm whether both marriages were in fact legal civil marriages. Its quite common for someone who was legally married, but separated, to have a religious only ceremony with different partner at later date - this was common in pre-divorce era Ireland whereby couples who had option 2 above quite often had the religious part of their marriage annuled by the Catholic church allowing the couple to have religious marriages other people in a catholic church wedding at a later date (though legally still married to original partner).