Hello Lesley
I am unsure why your question has dropped under the radar of the pumpers. Anyway here is my attempt to answer your query, albeit I should start by stating the caveat that I am slightly underqualified to respond! Although I use a pump I do not tend to use the extended bolus facility very often for myself, the control I get from a standard bolus is generally sufficient for my purposes. But I have an idea of the theory so here goes.
As you probably know the purpose of delivering the insulin over a period of time, called an extended bolus, is so that you have less insulin delivered at the beginning but then insulin continues to be delivered for the time period you select. The effect of this is that you do not have all the insulin active at the beginning and at least some of the dose is active for longer.
What you describe for your weetabix and banana breakfast - with the BG rising slightly after two hours but then dropping significantly in the next two hours - suggests that your initial dose is maybe a little too low and the dose is active for too long and maybe the overall size of the dose is too high, hence the end result is that your BG drops too much.
I think the reason for the drop is that your breakfast is unlikely to raise your BG for a period beyond two hours (technical term it has a medium glycaemic index - the banana is slightly low, the weetabix slightly high so together they are probably medium) so delivering your insulin over one hour probably extends its effectiveness beyond the time that your breakfast carbs are in your system, hence with no carbs to consume the insulin causes you BG to drop.
Basically if you reduce the delivery time then your initial dose rises and the time the insulin is active is reduced.
So in answer to your specific question yes reducing the bolus time to 30 minutes should help. In fact I would seriously consider delivering your full dose in one go for this breakfast. If you find that your BG still drops, as I suspect you will, then I would also consider changing your ratio so that you have less insulin for you breakfast, try say 1.0 units per 10g.
Blimey, this turned into a bit of long explanation, hope it makes sense and I have not confused you even more! These things are not easy to explain!
It would probably help if one of the other pumpers who use the extended bolus could also provide you with the guidelines they use for delivering the insulin over a period of time e.g. what type of foods they use this facility for and how they decide on the time period etc. As I said I do not consider myself an expert on this subject so it would be good if someone else could also give this a sanity check.