Bing ads - am I missing something?

I have experience with both Adwords and Bing, but it's a bit hard to answer your question without a few more details on the niche etc... I will make a couple of comments.

This is strange for sure, Bing is usually less expensive but not always. The general thought (which I have not seen as true) is that Bing traffic is better 'buyer' traffic. Usually more professional and better off people use Bing (apparently). This could be particularly true in your niche so people are fighting for that traffic more on Bing. But 10x more seems insane! I guess all I can intelligently do is say, I agree, something is VERY odd here.

First, it's not Bing or Yahoo or Google asking for it. It's the market forces as set by the advertisers. This could be very different on different networks but it is not typical.

You will typically get 15-20% of volume on Bing since Google is more popular. Because of that, less advertisers use Bing (although I think that's a mistake, 20% more is nothing to be ignored) and thus TENDS to make costs lower. But that doesn't mean there are less advertisers nor that costs will be lower on Bing.

Next, you say a £0.70 bid in Adwords gets you top position. But there's a difference between bid and actual CPC. Maybe you meant you pay that much. But if you bid that amount, what is your actual CPC? And if you BID £7.50, you surely won't pay that much. You could test it out and see the actual CPC you'd pay.

I would check out your campaign. If you used the tool to import your Adwords campaign into Bing, this could have not imported the way you think. Check campaign settings in particular. As with Adwords, stay away from broad matches, even more so in Bing. Are your ads approved? Don't increase bids just because someone at Bing suggested you do so.