No longer accepts cash. Only options are credit card over the internet or registering through the website and paying through SMS. Both options have a HIDDEN SERVICE CHARGE and you have to hand over your personal details. There's no option to opt out of marketing nonsense either. Would not use again.
I love this car park. It's easy to get to, it's linked directly to the Howard centre, you can pay by card or cash (although as the other reviewer says, you don't always get a receipt) and most importantly for me, it's number plate recognition operated, with no barriers - currently I drive a car where the drivers side window doesn't work and I have to avoid barrier car parks when possible. Some of the bays are hard to get into with a pillar on each side, but I still love love this carpark.
Be careful that the machine has accepted your payment and get a receipt (won't always give one) as it is confusing and you may think you have paid when you haven't! It happened to the person before me. Welwyn Hatfield Council have handed management of this car park to CP Plus who are very unhelpful and incredibly rude, not Local Parking Security (that's just an advert).