Notably was it the work of overall shifts in the world economy, the inadequacy of labour and conservative politicians or the large influence of unions as some seem to say? Obviously the answer isn't definitively one or the other but is there any one factor that contributed more than any other?
And a follow up question: To what extent did Thatcher fix these problems? Was it primarily because of her that the economy eventually improved or would it have recovered regardless of who was in power?
I'm afraid I can't address this question in more detail (at least until Saturday at the earliest).
However to start with I'd recommend at looking at the data to examine the basic assumptions. Here's total GDP and here's GDP/capita growth. In the first graph you'll notice that UK GDP doubled every five years in the 1970s. In the second you can see that growth was similar to the US and France, and heavily influenced by the two oil shocks.
The real shrinkage came in the first half of the 1980s as manufacturing collapsed. There's a good argument that had it not been for the boost of the Falkland war, the Conservatives would have lost power in 1983 because their economic policies did not appear to be working.
Of course, this is the high-level view and does not show problems like balance of payments, productivity, inflation or unemployment, all of which were significant problems and in at least some of which Britain performed worse than other comparable countries. To at least some extent the problems of the early 80s were the culmination of structural problems in the UK economy, although this doesn't mean that the government's policies were the right response. (For example, I've seen it argued that strikes were less of a problem than other European countries, and accounting and investment practices caused major problems for British industry.)