Don't worry, it's a strange one..
get onto the B3081 towards Melbury Abbas
The actually B3081 is a left turn, but if you don't turn off the road you are one you carry on to Melbury Abbas, then just carry on and you end up back on the A350
Hopefully if this work you will see the junction below
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The actual B3081 is the right road off to the left, if you carry straight on here you will be fine