Thanks.
I've just got to find a sleeping bag that's not over 60 quid and I'm sorted
At that price point, for the summer, I think it's just a case of taking a punt on one of the established brands and seeing how you get on. As warm, lightweight and compact as you can get away with, it's always a bit of a balance between those and budget.
One of the best items I got a couple of years back was a cheap down jacket from Amazon to keep in my sleep kit just in case the temps drop. You may even be able to get a cheap down quilt from eBay, we got some down quilts for about £25 each a couple of years back and they are perfect for warm-weather camping.
My main advice though would be to try your full kit out before you go, a night in your garden or something. Nowt worse than having an uncomfortably cold night and then knowing you've got to do it again the next night.
To add a bit of insulation to the Numo you can use one of those mylar survival blankets, or cut a length of that thermal bubble wrap stuff you get for car windscreens and put it under the mat. Cheap and effective.