What does it all mean?
Good question. I'm glad you asked. First and foremost, the data released by the seven lenders - Barclays, Clydesdale, HSBC, Nationwide, Lloyds, RBS and Santander - means that, for the first time in Great Britain, we can get a sense of how mortgage finance is distributed at a fairly local level. This doesn't mean that we know how much each household owes or how much is owed in each postcode sector. That's because, as the Council of Mortgage Lenders press release said at the time of this data becoming available, this data only covers 73% of lending. The total amount of lending accounted for by this data release is just under £900 billion, whereas the total size of the UK mortgage market is about £1,266 billion.

I thought it would also be useful to provide a little additional contextual information in the map pop-up windows so I have divided the total outstanding mortgage loan figure for each area by the number of households in the 2011 Census who said they had a mortgage. This is clearly far from perfect because of the different time frames and incomplete lending data but it does give us some idea of the local situation with regard to household mortgage debt. However, I've only really provided this for a bit of context; I certainly wouldn't use it for any formal analysis at the local level.

What can the maps really tell me?
As I've said above, it's best not to read too much into any postcode sector averages in relation to individual household debt but the maps and data for individual postcode sectors can provide us with at least two pieces of information that we previously didn't have. 1. We can get a sense of the mix of different lenders in an area; and 2. We can see how much the above lenders have invested in individual areas. These will be affected to a great extent by local property values and by the total number of people in each area, and also the overall size of the individual lenders. 

In the example postcode sector below (CF24 4) we can see that Lloyds accounts for a large proportion of all lending in the area and that the total value of mortgage loans outstanding at the end of June 2013 was nearly £118 million. I have also included information on the total number of households in each area, as you can see from the image below indicating that CF24 4 has 839 households.

How much is owed to each bank?
To provide a bit of context I've embedded a chart below which shows the percentage of all lending owed to each lender (out of the total of over £800 billion owed to these seven banks). You can click on the individual bars to get the precise percentage figures but as you can see, Lloyds is the biggest mortgage lender by some way. The Lloyds Banking Group acquired HBOS and Lloyds TSB in 2009 so this is part of the explanation. Clydesdale is a Scottish bank and it accounts for the lowest share of total lending, at just under 2%. Since 1987 it has been owned by the National Australia Bank. The figures include lending by the Yorkshire Bank, a subsidiary of Clydesdale. Nationwide, which is actually the UK's largest building society, accounts for just under 15% of the total.