@Paddy Your pacing plan on race day should be the same as your pacing plan in training. If you use HR in training, use it in racing. And, of course you’ll use what you learned in training and apply it to racing. In your case, you learned that you experience cardiac drift (which is normal) in your long runs, and that HR and Pace start to become uncoupled. This will allow you to make the decision to maintain a certain pace even when HR drifts upwards, because you know it worked in training. Since you also used Pace in your training, I’d rely on Pace as a primary and HR as a secondary. It’s still a good idea to set an upper limit on HR for a marathon, say upper Zone X or Zone 3, even with normal cardiac drift.
The temperature also absolutely played a part in your elevated HR. This is another normal development using HR in increasing temperatures.
@Dean You’ll notice that not only do the long marathon plan runs give you “permission” to go into Zone X, our Race Pacing guide provides the same range for a marathon. It’s a matter of risk/reward and experience level to decide to wade into Zone X, and should certainly be tested in training first. If your 16-mile run was done in Zone 2, I’d stick there, but a small drift into Zone X after a taper is a limited risk.