Julie
I cannot give you a definitive answer as we cannot give detailed answer to financial questions on the forum, if you are concerned that the amount is incorrect then you need to get face to face advice from a qualified advisor who can go over the figures with your. Do an internet search for "welfare advice" with your postcode, town or county.
Standard practice is to disregard benefit back payments for 52 weeks, if your payment was made more than a year ago then this payment will now be treated as capital. See
Asset rule for ESA(IR) If they are deducting £20 per week then this would imply that you have savings and assets of about £11,000 (£6000 + 20 x £250 = £6000 + £5000 = £11,000)
Your partners ISA will be included in the calculations.
Gordon