HO apparently accepts 30/hrs week or 120hrs/month as full time.. check the application form.
Technically 30hrs/week calculates to 130 per month
Anyways..
Have you considered increasing employee B hours to full time till 31st March. Reduce hours of employee C. And add employee D for a month f/t or if part time-combine with employee B.
Also consider using weeks from 27/03 to the date of application.