That is a good time on that course! As are Rob's, on any course, for that matter.
I did the E2 twice but both times were windy days so didn't get a decent time. The first time I got beaten by @Karla. I think they've made it faster now, and it is hard to get on, so would need to be a long-term project, getting an LTS somewhere else even the Hounslow fills up now, and with the F1 having gone, that is actually a bit tricky.
My pb is on the A50 / Etwall course. I got a short 4:10 on a good day, but lost some time as I did an extra lap of the first roundabout, and carried one bottle more than I needed to (excuses!).
I have seriously considered driving down to the A31 in the middle of the night with a bucket of cement and a trowel...