Looking into systems i think that an unvented system will be the way forward in the future - the mains appears strong (not measured yet but pretty certain it'll be good for the unvented), a combi just causes too much issue especially when I'm cleaning the car and she's showering! We also have a very low pressure mains fed stat shower in the en suite which is virtually unusable due to rubbish pressure
Those two statements are contradictory.
As for a combi being no good while, by implication, an unvented cylinder will be OK, that, to put it mildly, is not true. Both are mains fed, so you will have exactly the same problem cleaning the car while she is showering whichever method you use.
You need to check the incoming flow rate and pressure before making a decision. It's easy to do.
If you have a garden tap (normally fed from the mains - check first), measure the flow rate using a marked bucket and watch.
Now repeat (at garden tap) with the kitchen cold tap turned on fully at the same time.
If there is no mains fed garden tap, just do it at the kitchen tap; but be aware that some taps restrict the water flow, which is why the garden tap is preferred.
You need a minimum flow of 20 litres/min for an unvented or a combi.
How well is the house insulated? Double glazing? Cavity walls? Loft? You can get grants for walls and lofts - check out your gas/ electricity supplier.