It's not a disaster if you don't get the SEP class rating signed in time, but it will cost you a bit and give you some extra hassle.
The deal is that you must meet the 'by experience' flying requirements (including flight with instructor) AND have your class rating signed before the lapse date.
If you do not manage to achieve BOTH of these requirements then your licensed has lapsed. In this case you have to undertake a skills test with an examiner (although there is no nav element). This, of course, means you will have to pay an examiner's fee on top of the plane hire.
Hope you get it sorted.
PS I don't want to sound like me muvva with the 'told you so line', but remember that you can do the dual flight with an instructor at any stage within the last year of the licence. Get it done early and you shouldn't have to be at the mercy of the weather in the last month. Good luck