dg wrote:
I want a system for my toolbox to sharpen chisels on site - so no bench
grinders!

Angle grinder!

I want it to put a finer edge on chisels in use, and not for
resharpening any realy blunt ones.

The choice seems to be between a honing guide and an oilstone, or the
Trend Fasttrack system.

Can anyone recommend a particualr one? And is the Trend system worth
the expense?

I liked a simple diamond 'stone' - medium grit.
It's gonna depend on what you want the chisels for.