You'll be off the treadie soon Rachel, deadly dull but worth doing.
This weekend is the end of week 5 of your 18 week plan. Here's the original plan for weeks 6 to 10:
Wk Mon Tues Wed Thur Fri Sat Sun Total
6 Off 5 4 5 Off 13 3 30
7 Off 4 5 5 Off 20 3 37
8 Off 6 6 6 Off 12 5 35
9 Off 6 4 4 Off 20 3 37
10 Off 6 6 6 Off 12 5 35
Week 8 is now where you have your half marathon on the Sunday, so that just shifts that weekend around (just a light run on the Saturday, race on Sunday). Since you can't do your long run this weekend, there's a question of whether you should just skip it, or move it to next weekend. That would mean successive weekends of 18, 20, race, 20, which is a tough series but would set you up nicely with 8 weeks to go. Downside is a risk of overdoing it, but as you're running your long runs slowly you should be OK. Lets see how you get on when you get back on the roads next week and then decide what the weekend will be.