Travelodge Horsham (4.1 miles)
Premier Inn Horsham (4.6 miles)
Premier Inn Horsham North (4.9 miles)
Travelodge Billingshurst (8.6 miles)
The Windmill Inn West Grinstead (4.9 miles)
South Lodge Bolney 5* Hotel (7.1 miles)
Of course many other hotels and B&B’s are available on sites such as booking.com or Trivago.co.uk. Average price that I've seen ranges from £75 to £150 per night.
Well, except South Lodge, that's £571 per night which includes breakfast and spa access if you really want to treat yourself to a bank holiday retreat!
If you don't need access to electric hook up, you could probably get away with stopping on the Southwater Business Park road overnight.
There are several camp sites in the area so that may also be worth a look.
Camping is not allowed within the grounds of the country park, there are however camping can be booked nearby at Sumners Ponds.