Digger Dave,
If you calculate the required hot water load and the initial ( early morning ) heat load it will give a reasonable indication as to how the flow rates may be divided. Without wishing to consider for you to spend more money - look at Honeywell Evohome. It’s expensive ( £700+), but is very effective at simply zoning ( up to 12 zones) which makes it easier to optimise a heating / dhw system with a boiler that is slightly undersized.
I use it very effectively on a heating system in an old uninsulated house with a 25kw boiler feeding a 210 litre unvented tank and 18 radiators. The Boiler is (in my view) undersized by 5kw - but Evohome has enabled me to tweak an optimise the system to such an extent that it is now an effective system.
Hope this helps