Hello,
A similar thing has happened to me, I committed a speeding offence (36mph in a 30mph zone) I returned the information and driving license number within a week and was then issued a conditional offer of the speed awareness course or 3 points an a fine - obviously I wanted to do the speed awareness course.
Similar to yourself, I read the letter to say the course had to be booked within 5 months of the offence, my offence was on the 9th Jan and I could only book it today (17th Feb) due to having an operation recently. I have booked on the course but I am worried it was past the 28 days and I may receive a summons.
May I ask what the number was that you rang to explain your situation? I am very worried as I do want to do the course but I read the information wrong and thought I had longer to book it than 28 days!
Any help would be appreciated.