I am going to apply it to a host of Asters this year and see how it goes.

I often chop back the Geranium and they thrive on it. Tho' this year they are a bit later coming through so I may not hack at them this year.

Certainly not going to try in our garden of alpines and woodland plants!

Yes, you can cut back perennials by about half to delay the flowering period.

The Chelsea chop so named because it is usually done around the end of May The Chelsea Flower Show time and is a pruning method limiting the size and control the flowering season of many herbaceous plants.
I use it a lot in my garden. First will be some of the bigger Phlox subulatas and then Tumbling Ted.

I tried this last year on solidago and sedum spectabile. They hadn't got any flower spikes yet. I have several clumps of both of these, so tried it on half of each. It works fine - it delays the flowering period by a few weeks, and reduces the eventual height of the clumps. The only thing was - I grow probably more of these than I would otherwise because of their attractiveness to insects. The solidago attracts hordes of bees, and the sedum butterflies (especially peacocks, I find). I felt that delaying the flowering - particularly in the case of the solidago - was not very useful to the insects. I think I'll leave it this year. From the point of view of extending the flowering period, and restricting the (not inconsiderable) height of the solidago, it was actually successful.

I also use it on solidago and sedums and on the large clumps just cut back the front half of the clump, that way the back of the clump flowers first, then the front a bit later. Worked well last year. I'm going to try it on phlox as well this year.

Thank you very much everyone - Its much clearer now and very useful to know. But as my perennials are mostly newly planted I think I'll wait until another year. Cutting half the clump sounds like a good idea - might try that on some older very thuggish geraniums though and see what happens.