Not necessarily month by month, but since you bought it it will have immediately dropped maybe £3-4k, if not more, and will then typically drop between £100 and £400 per month. With a large enough deposit to counter the initial hit a payment of £300 ish per month would probably keep you out of negative equity.
The value of the car will fall sharply at first and then more slowly. Your payment is geared towards equity at the end of the agreement, I would guess around 36 months, by which time depreciation will have slowed and you should owe a similar amount to the value of the car.