When i had an email from Onmarket regarding changing...

When i had an email from Onmarket regarding changing allocations, from an allocated amount to zero, they said there is a potential for shares to be sold that you don't own.
The consequence of which is that you would have to pay to buy back shares in the open market to cover your 'short' position with additional fees.

So I would very much advise people not change allocations in the future.