I have 2 locked gates one at each end of my drive, if one is unlocked can they enter the driveway?

Yes

A bailiff my enter the debtors property peacefully by opening the unlocked gate.^[1]

The bailiff cannot put his hand through the gate to reach inside to draw back a bar to open the gate because that amounts to burglary.^[1]

If the key is in the lock, the bailiff may enter by turning the key.^[1]

If the bailiff finds the key under a mat, he may not enter using the key,^[2] or enter using a landlord's key.^[3]

Bailiffs may enter property by any usual means of entry,^[4] and must only use a door or usual means of entry to enter premises.^[5]

These rules do not apply if the Bailiff is collecting unpaid court fines, or money owned to HMRC.^[6]

^{[1] Ryan v Shilcock [1851] 7 Exch 72
[2] Welch v Krakovsky [1919]
[3] Miller v Curry [1893]
[4] Regulation 20 of the Taking Control of Goods Regulations 2013
[5] Paragraph 59 of the Taking Control of Goods: National Standards 2014, April 2014
[6] See Bailiffs and entering using a locksmith}