I'm not sure I can answer your question really but, theoretically you can put in 100% of your pay into your pension/AVC's up to the £40k limit. But you also have to take account of other deductions from your wages, such as your basic 6% contributions and NIC's which will reduce the amount down. Although the numbers you quote seem low to me.
Any extra money you can't pay into AVC's for whatever reason RM give, could go into a separate personal pension, SIPP, etc.
On a separate note, I never realised you had to fill in a form to change your AVC contributions. Every time I've changed the amount I pay, I've just send them a letter saying I want to increase/decrease by £x to a weekly amount of £y. Never had any problems!