Right, I have a 28 and 40 dual chainset and I find I miss my really low gear that got me up all the slow climbs,

What I wondered was, can I change the granny ring for something smaller 26 or 24 tooth or will this ruin the shifting?

Yes, <I think> the difference should be no more that 12 teeth between rings.

Have you considered getting a 11-36 rear cassette? According to Sheldon Browns gear calculator, the ratio is very similar to a 22 front and 32 rear, which should bring you back to what you want for the climbs.

Would changing the cassette solve the problem?
I did it on a road bike with very pleasing results.