HI Natzwizz
You need to phone them ASAP to see if they will give you any more time. Some people have been granted another further two weeks. There are a lot of problems with the post. You probably don't want me to say this now, but always give yourself some wriggle room. If they won't grant it, yes there is a risk of the claim being closed and you having to start again.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems