I have 4 bikes on an Aviva multi-bike policy which costs me around £300 including all the fully comp knobs and whistles, European breakdown included.
One bike is 44yrs old, like yours, and while it is included at a declared value you tend to find the specialists can often cover these vehicles for very small sums. I did look into covering that bike separately along with another similar age bike (currently not insured) with a specialist classic bike broker, and the 2 bikes would have come in at just about £100 together. If it is a particularly valuable bike then a specialist is probably the better route with an agreed value, which they are used to handling whereas the routine insurers deal with book values usually.
I was looking at using
https://www.peterjamesinsurance.co.u.../classic-bikes
who I believe was originally connected with
https://www.footmanjames.co.uk/class...SAAEgI1PfD_BwE
I have no connection nor experience with either.