In article ,
says...
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The house is a semi from the 30's if it's any help and a bad ascii
diagram below shows the setup of the joists

You need to use a fixed width font like Courier to make ASCII art work.

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A structural engineer spared me a couple of minutes on the phone and
advised the joists should be ok to hold the weighth of the boards
(although the celing below may crack but should not collapse), also
advised agaist screwing boards into joists if possible as they may crack
and distributing light storage weight into the sides.

Is this sound advice and is there anything you guys can think off that
may help the situation or provide reassurances. WOuld writing off the
cost of these boards be wise and just replace with ordinary loft bvoards
that can be bought in B&Q and the likes.

While the boards you have are heavier than the nasty little ones you get
in the sheds, they will probably also spread the load a bit better when
you are walking on them, and that's when the load will be highest. As
he said the weakness will probably result in flexing rather than
collapse, and if you're lucky this won't show through the new
plasterboard. As you already have the boards why not lay them and see
what happens when you walk on them - listen for creaking, feel for
movement and have someone downstairs to shout if a crack starts to open
up. If it survives that then store stuff at the edges or over the
middle wall.