I have had a new shower fitted and the water drains away fine until I wash my hair and create bubbles then it startsto back up!! Why??

Are yoj sure that the back up is not caused by hair covering the drain as you shower.

When you finish your shower do you have hair covering the little holes in the drain cover.

I don't hae a good adea as to why the bubbles from the shampoo would cause the back up.

A slightly clogged or undersized drain pipe can become saturated with foaming soap and get even slower. Try less shampoo or get your pipes snaked/rodded. BTW guess what usually clogs the pipes, soap scum and hair.

Bob's on to something here. I think it's the bubbles from the shampoo that you see and not a back up out of the drain. I use body wash and that added with the shampoo as I rinse will give me bubbles coming out of the drain. Now if you have water backing up out of the drain then the drain needs to be snaked. Regards, Tom