From Wells:

Take the A39 out of wells sign posted Bath Bristol, at New Street Junction turn left onto the A39 Sign posted Bath, at Bristol Hill Crossroads continue forward onto the A39 Sign posted Bath, at Pen Hill Junction bear right Sign posted Bath, turn right onto Turnpike Corner Bear left onto Green Street.

From Shepton Mallet:

Take the A37 sign posted Bristol Bath into and through Ston Easton, (do not turn left on to Green Street there is no access to Mendip Plains EC along this road) at Rush hill junction turn left on to A39 sign posted Wells, turn next left onto Turnpike corner and bear left onto Green Street.

From Bristol – Avoiding Clean Air Zones:

From Bristol M4 take junction 19 south on to the M32 Bristol.
Travel for approximately 600m and bare left on to the A4174 (ring road) heading for Keynsham.
Continue on the A4174 for approximately 8 miles stay on the ring road until you get to the Hicksgate roundabout, turn left on to the A4 Bath Road dual carriage way signposted Saltford/ Bath.
At Broadmead roundabout take the 4th exit on to Bath Road Keynsham B3116 continue for approximately 700m and at the 2nd mini roundabout take first exit on to the B3116 Wells Way.
Continue for several miles to the two headed man where you will turn right on to the A39 towards Marksbury.
Bear left at the traffic lights with the CO-OP fuel station on your right-hand side.
Continue on the A39 where you will get to a T junction controlled by traffic lights with an ‘EXPRESS’ hand car wash in front of you. Turn left on to the A37/A39 signposted Wells/ Shepton Mallet.
At traffic lights continue straight ahead and up Rush Hill at the junction continue forward onto the A39 approximately 1 mile (sign posted Wells) take the next left onto Turnpike corner and bear left onto Green Street. Mendip Plains first left.

From Bath – Avoiding Clean Air Zones:

From Bath M4 take Junction 18 (Tormarton roundabout)
Proceed south on the A46 for several miles before getting to the roundabout (Cold Ashton), take the third exit on to the A420 signposted Bristol continue for several miles through Wick until you get to Bridgeyate double mini roundabout.
Take the first exit left on to the A4175 Bath Road continue on to Oldland Common, go straight over the first round about (garden centre on your left) then at the second roundabout take the first exit to Bitton A4175.
At the next mini roundabout take second exit (right) towards Keynsham/Bristol on the A431.
At the next round about take the first exit to A4175 Keynsham Road.
Continue on until passing Keynsham railway station on your left when you get to the next set of traffic lights go straight ahead to the mini round about.
Next round about take the first exit left to Keynsham Highstreet (this is a one way system useful to know for your return journey)
At the end of the high-street turn left down Bath Hill B3116
Continue straight over two mini round abouts up a hill to another mini roundabout then take the second exit to the Wells Way B3116.
Continue for several miles to the two headed man where you will turn right on to the A39 to Marksbury.
Bear left at the traffic lights with the CO-OP fuel station on your right-hand side.
Continue on the A39 where you will get to a T junction controlled by traffic lights with an ‘EXPRESS’ hand car wash in front of you. Turn left on to the A37/A39 signposted Wells/Shepton Mallet.
At traffic lights continue straight ahead and up Rush Hill at the junction continue forward onto the A39 approximately 1 mile (sign posted Wells) take the next left onto Turnpike corner and bear left onto Green Street. Mendip Plains first left.

For any additional information around the clean air zones affecting Bristol and Bath please follow this link:

https://www.gov.uk/clean-air-zones