There's no conclusive evidence that gait analysis provides any real benefit so unless you are adamant that it works for you it probably isn't worth bothering with... Either buy a sale/discounted pair from a store or a cheap online deal if you know what you'll be buying will be comfortable.
If you've negotiated £800 then just go for that... I think the LOOK website is now stating £1000 as the (unnegotiated) amount to be raised... I expect all their places will eventually go; a grand isn't that much to raise for anyone who is that bothered about doing the VLM.