Hi Rodders56,
As well as less basal just before and during the exercise you could experiment with how you eat before it - in the meal or snack before the exercise have less bolus than you'd normally take for it.
It's all a bit of experimenting. For me, I've started cycling a bit recently too and 50% lower background during the ride then check every 30/45 mins or so and if BGs go below 5 (ie before going low), then have a small snack / couple of sweets. Previously I used to not reduce the background and just have a banana beforehand (without bolus) and that worked for shorter exercise. For longer exercise, am more likely to go low a few hours later so need to reduce carb ratio at the next meal too. All depends on intensity and duration of exercise.
The dafne course has good advice on it (and I think there's refresher on exercise), if that's something your diabetes team can get you onto.
The runsweet.com website encouraged me as included discussion of how through trial and error others have managed blood levels during a marathon.
Hopefully you'll find something that works for you!