[Growster...]

We've had the old thing for ages now, and it really does need some TLC, especially as once the bindweed sets in, there's not a lot we can do! It's about seven ft by four ft, and against the North face of a low brick wall

When would be the best time to cut it back, and would it be wrong to take it right back and let it all start again?

It's a beauty when fully out, so just a year without blooms would be a bit of a gulp, but not the end of the world!

[mumofstig]

If I were you, this winter, I'd take out (it will be fun untangling them  ) about half of the main branches from low down on the plant, plus any diseased or weak stems elsewhere.
That should make it grow some new stems from low down, but you'll still get flowers from the old growth next year.
Then next winter, assuming it is recovering and growing well, you can prune out the other half.

[Growster...]

Sounds like a perfect plan, Mum!

I don't think I've ever got close to the roots, it's too dangerous!

[New shoot]

This does look a bit tatty for a few weeks but it works.  Cut the stems you want to prune, but don't untangle them.  They wilt and the die off so you can track them up the plant and cut sections out as you go.   That way you only pull out short lengths and avoid dragging out long bits and potentially damaging stuff you want to keep.  You will be able to see and can then remove any additional weak or sickly looking stuff.

From the New shoot book of lazy and quick fix gardening 

[mumofstig]

That's a good'un! 

[Growster...]

Marvellous!

Many thanks Mum and News, we may even start a bit earlier, as the poor old thing is in such a sorry state!