Hi johnr,
Just so you know, I made a Lancashire yesterday using liquid rennet (750 ml strength) that had an expirey date sometime in 2009! It's about 6 years past it (apparently). The label says use 0.7 ml/10 Litres, which I found was too strong when first started using it and backed down to 0.6 ml. I used 0.6 ml yesterday, and it floc'd in about 20 minutes (11 litre make). Bit slower than usual, but it was more the final firm setting that was slow as things were looking to be on time but the surface never quite gelled to leave a ring. Cheese make went perfectly fine after that.
In other words, don't worry about it. Rennet may lose strength over time, but if it does, it's very slowly.