Hi Neil
I stopped working with the HC-SR04 on Zumo + Explorer HAT Pro and moved to the Sharp GP2Y0A41SK0F which is very reliable. Plus, as an analogue device it gives me:
a) something to plug into the analogue pins (freeing up the digital)
b) experience of reading and interpreting an analogue voltage signal.
I like the Sharp over the HC-SR04 as no time-accurate readings are needed, which is always going to be a little iffy on a non-realtime device like the Pi. I go into using the Sharp in a fair bit of detail at http://rasptut.co.uk/files/zumo-george-part2.php . Whichever sensor you go with hopefully my (only two so far... more to come) posts on using the Zumo with the Pi will help. NB: the A+ is about 1.5mm too big on each side and I'm thinking of swapping to the Zero. Who would have thought the A+ is "too big" ;)
I do recall that Pimoroni were going to look at improving the timings to try to get things working. Although they did suggest to bypass the Explorer code and use RPi.GPIO direct may help. Sorry I can't be more help on the HC-SR04.