I have once, middle of last year, after we got the late-opening night wrong. Parked in the layby right at the bottom of the access road (wasn’t ‘No Parking’ at the time as I recall) & cycled up to the reservoir & picked up the ‘official’ trail.
There’s nothing preventing you accessing the trails, i.e. no locked gates to climb over. We also met a few local dog-walkers on our way up to the reservoir.
Technically it probably amounted to trespassing as it’s private land.
The website does state “Parking outside the centre in anticipation of a late evening ride is not permitted. Expansion of the trail network will be jeopardised if you fail to comply with centre opening hours.”
You could probably get away with it, but if the landowners catch on & think it’s a problem then it could cause issues for the future of the site.
EDIT: unless it’s all bluster for insurance reasons