Loft insulation and joist depth

[ColinM50]

I keep getting calls from these "energy advisors" telling me I can get a free top up of my loft insultion and when I ask them what to do about all the stuff in my loft which is boarded out, I'm just met with "hmm, you'll have to empty the loft before we can do it". Which isn't much help and really means I won't be able to store stuff in the loft anymore. All they've suggested is that they'll rest the stuff on top of the new insulation. which is no solutiojn at all.

I'm sure others have had the same problem, so what did you do?

Existing joists are 75mm deep and we've got 75mm of insulation and boards down resting on the joists. I'd really like to increase the insulation to 270mm as recommended now and I've thought of making up some dummy joists as the pic below. So sort of 50x50 timber top and bottom with the metal webs between and screw/nail the bottom beam across the existing joists at 15 inch centres and then after the insulation's been put in, screw the flooring on to the top 50x50. Does that seem sensible?

Now comes the real question. What's the metal web called and where can I buy it? Tried a couple of builders merchants locally and they all suck their teeth and tell me I'll have to have the joists made up properly but can't help other than that. Anyone know please or get any other ideas?

[ColinM50]

Thanks VX and 944, but what's the name/cost/supplier for the metal bits so I can make these dummy joists and get the bods in to put the insulation in?