From the website:

BECOME A MEMBER OF CLUB TRESemm AND RECEIVE A FREE SHAMPOO AND CONDITIONER SACHET

Recycle your shampoo bottle in our in-store shampoo recycling bin for FREE solid shampoo tailored to your hair type!

If you do live near one of the stores, you can buy something off their website and write a brief description of your hair type in the comments box and they will send you a sample.