Getting there late on Saturday.

[BensGrandad]

Without doubt do this. Avoid the top of the M23/M25

Come off the M25 at Leatherhead, A24 to Horsham, from where I would continue down to the 'Buck Barn Crossroads' , left there across the A272 to Cowfold/Bolney and down A23 to Mill Road.

Which will be full by that time. So left at Patcham and along the A27 to Mithras House.

Or through Reigate on A217 and join M23 at Gatwick but sometimes that gets held up before the junction with traffic going on to the M23 but not as bad as the top of M23/M25