Hi guys.
Glad to say i eventually got this issue sorted.
I asked a local tiler who has a lot more experience than me to come and take a look at it with me. He took one look at it and was able to name the sealer i had used with out me telling him.
His solution was to use cellulose thinners to remove sealer, (I had my doubts but it worked a treat) it took a bit of time to do but brought the slate back to how it was before the fiasco. I then resealed with Lithofin slate sealer and i have to say its a much superior product. It looked very, very glossy when applied but dried out with a nice finish.
Customer was delighted with the end result, it took a few extra hours labour and a few extra quid out of my own pocket but sure it's all about keeping the customer happy!!!!!
Thanks to everyone who replied, it was much appreciated. I have a few photos and i'll get them up when i get a bit more time.
Thanks again
Ruairi