So how can I dig them out in the 20's? It appears the Tories were solidly in control during the period with the Liberals declining and Labour still finding it's feet so how possible is reform? With 20/20 hindsight, particularly from a left of centre POV, the need seems obvious, but how apparent was it too politicians at the time and how much chance did they have of achieving not just reform but the right reform? Were the Tories of the period as hidebound as I'm thinking they were?
Landshark
Depends on you're POD. Something a bit earlier makes it a hell of a lot easier than something by about 1920.
As Lord Douglas said it wasn't totally the Tories fault. For instance Labour formed a minority government in Jan 24 because the Liberals supported them rather than the Tories as the latter were committed at the time to tariffs and both other parties opposed them. If you could have a clear Tory victory [me saying that

] or for some reason the Liberals support the Tories then you might get tariffs in earlier which,
if used properly, I would say could ease both financial and economic problems as you might see some revival of British industry.
However really need something a few years earlier. Avoiding the distraction of the Greeco-Turkish conflict and the intervention in Russia would allow military spending to be cut a lot earlier, as near wartime spending on the navy continued for a couple of years.
Most of all some of the immediate post-war ideas for reviving British industry and society could be applied but how to get them in. The Liberals are split between Lloyd-George and Asquith at this point so cease to be a unified factor until its too late. Also while they have some radical ideas that would help they still have a strong element opposing more government intervention. The Tories are the Tories and will generally oppose drastic change or allowing more opportunities for ordinary people [since the latter requires investment and hence taxes]. Labour has some ideas but also idealogical baggage that makes it difficult for them to unify the country rather than divide it further.
I think you would need to go back to at least mid-late WWI to have much hope of squaring that circle but then so much else could change. It all depends on what sort of world you want to develop.
Even without major reform of society and the economy Britain will still be one of the great powers and, presuming the US slips back into isolationism and with Russia prostrate by the war and the communists it will be the major one in most fields unless/until the next major challenge comes along. This gives opportunities for further improvements but something needs to generate the will and unity for change.
One option with less butterflies might be to sacrifice the 20's and have them pretty much as OTL. Possibly a few changes as I presume you're thinking of a TL with no or modified Washington Treaty. Also possibly some resolution of the gold standard issue.
While I disagree with Phil on Britain's wealth at this period, see debate elsewhere, to get a consistent programme extra spending is probably required. This could improve some things by inadvertent Keynesian-ism, with money being cycled through some of the poorer northern regions that were economic stricken by the 20's. Also, if new construction is seen as necessary, say due to a new naval race, the challenge is where to find it and there is plenty of wealth available if there is the will to tax elements of the wealthy more. This could help unify people against the status quo.
Then say the depression hits, which is virtually inevitable with the shape of the US economy and nature of war debts. The butterflies mean that the Liberals are in a stronger position and come through the centre with clearly Keynesian ideas, as actually suggested in 1929 I believe, and are able to prompt major changes to bring the economy out of depression by investment in infrastructure. This starts off being to get the economy moving again, has a period when its targeted more at improvement in 'human capital' by spending on education, health etc, then increasingly gets moves into military spending as the threat of fascism grows.
Anyway, a few ideas that might be of use.
Steve