+1 for garmins, or even better a GPS logger you can leave in a jersey pocket, I reckon my £20 one off ebay was even more accurate than my edge as it had a better view of the sky (someone else did some testing with a forerunner on their wrist with similar results).
Doing a Sportive tomorrow that uses timing by Strava – the organiser can’t be arsed with all the transponder stuff and doesn’t really believe in Sportive Timing anyway so all he does is put a few segments in on his routes.
Why pay for the sportive then? Park in the same place the next day, without having to fight for a place, ride the loop navigating by GPS, then spend the £30 on as much cake and coffee as you can consume at regular ‘feed stations’ and be glad it’s not SiS and half bananas.
I enjoy sportives, but I’d not pay for one!