Seany, what a great bunch of replies you have received. This is why I have such a warm place in my heart for the fellow Catters out there. Excellent replies.
Being a whistle player for almost half my life (I'm 51), I can tell you from personal experience how to ACCURATELY tune those whistles that you have. Place them carefully on the ground, preferably on a nice piece of cloth, in a row. Then run them over with your car. Then, go out and buy a better whistle.
The posts here are accurate and very truthful. Cheap whistle = cheap sound = out of tune. Now I am not advocating that you go out and get a top of the line Copeland or Burke (which will mostly solve your problems MOSTLY) but some of the tips offered by the folks on Chiff & Fipple are excellent ideas. Also, a mid-grade whistle that may cost between $30.00 and $100.00 may be an expensive answer, but you'll find it to be more in tune but not exactly accurate.
Brian hit it right on the nose. Whistles are not accurate instruments by nature. Try to get as close as you can by matching up with the chief player in the session. And Les From Hull expresses both accuracy and etiquette by saying that tuning to the box player or piano player in a session is better than getting your "A" from an instrument that is variable in pitch, like a whistle (especially if it doesn't have a tuning slide).
Try to get into the world of tuneable whistles, and I don't mean tuneable by melting the glue off the plastic fipple. And please use your electronic digital tuner sparingly. If the whistle is a D, then get the A note bang on and then, as my cousin Tom would say in Dublin, feck all.
Good luck to you.