Mackie in "megalith builders " suggests that Monamore on Arran was the oldest but that was in the 1970's and the calibrated dates from charcoal from the site were 4350 to 3500 cal BC ., done in the mid 60's and considered imprecise for various reasons by Mackie who excavated there.
The most recent paper on Coldrum was last year "Dates, Diet, and Dismemberment: Evidence from the Coldrum Megalithic Monument, Kent . "
Michael Wysocki, Seren Griffiths, Robert Hedges, Alex Bayliss, Tom Higham, Yolanda FernandezJalvo and Alasdair Whittle . Which suggets "The oldest remains deposited at Coldrum represent
people who most probably died in the late 40th–early 39th centuries cal BC (3960–3880 cal BC; 68% probable " and "Posterior density estimates from our preferred model suggest that the monument was first used in 3980–3800 cal BC (95% probable) or 3960–3880 cal
BC (68% probable)."

Coldrum looks to be the oldest .