Hello Alison.
How about this then.?
Leave the M6 at Shap ( J 38 ) and join the A6. Enjoy the lovely drive over Shap Fells to Kendal and thence to the Caravan Club Site at Low Wood ( as suggested by John ). The following morning, you will be about half a mile from the A591 ( a dual-carriageway ) which in another three and a half miles will put you back on the M6 at junction 36. Apart from the short approach to Low Wood, this diversion will add just one mile to your journey compared to staying on the motorway.
In addition, by arriving earlier than you would at Garstang, you can enjoy the pleasant drive along the edge of Morecombe Bay to the lovely little village of Arnside where you can sit and watch the sun set over the bay with the Lakeland hills in the distance. Where to sit.? Simple, at the tables outside the Albion pub on the seafront. What to eat.? Simple again, the Albion does food but better than that, a short walk back along the front is one of the best chip shops for miles around.
If you don't fancy that, then there's the Wardens Secret which they didn't tell John about because they want to keep it to themselves. Leave the caravan site, take the first right into Nannypie Lane and in less than a mile you will come to the Strickland Arms a refurbished pub where the food is good and reasonably priced.
Hope this helps,
Olly.
PS - If you want to look at the suggested pubs, take a look at .
http://www.storth.com/pubs.htm