Park just past the Hill Inn. Walk up the path towards Ingleborough. After 10/15 minutes the path takes a sharp left and crosses a up over a small rocky ridge. The path crosses a wall not long after. Go through the gate and the follow the wall on the right (leave the path). The path continuing up to Ingleborough is paved from this point on so its easy to recognise. A few minutes walking brings you to the corner of the wall with a stile. Do not cross the stile, instead follow the wall to the left. A second stile a minute or two away is your destination. Across the stile is the huge obvious mere entrance of Mergeill. On this side of the stile, in an obvious shakehole, is the smooth, tight crawl to the aven entrance.
Meregill responds very badly to wet weather. However given a dry spell, a small amount of rain before a trip hand is unlikely to saturate the cave. As long as there is no rain on the day you cave then it you should be able to make a judgement on arriving.
The aven entrance is quite tight. Larger cavers may struggle to enter or exit this way. If in doubt try on the way in (before trying to use it as an emergency exit). The mere entrance is nicer but given the small amount of rope and proximity of the pitch to the surface it may be worth rigging the aven as a backup (just in case).