[theakston_uk]

Should I go for a basic ph testing kit like this

http://www.amazon.co.uk/Gardman-Soil-Testing-Kit/dp/B000ZEHBNA/ref=sr_1_2?ie=UTF8&s=garden&qid=1243069021&sr=8-2

or something a bit more indepth

http://www.amazon.co.uk/Nortene-Easy-Use-Soil-Testing/dp/B0013A1IP2/ref=sr_1_1?ie=UTF8&s=garden&qid=1243069058&sr=8-1

[Salmo]

I would go for the Gardman.

The pH test with barium sulphate and indicator liquid is well tried and reliable. It is the standard test used by professionals. It is more reliable than many of the electronic devices on offer.

Testing pH is by far the most important test for your soil and 15 tests should be sufficient to get a good picture of the lime requirement of an allotment sized plot. Look up liming in the information section of this site for a guide to liming different soils.

Tests for nitrogen, potash and phosphate are seldom of much practical use. I can see little need to know the level of nitrogen and you usually know this by the way plants grow and the amount of organic matter in the soil. By far the biggest influence on the availability of potash and phosphate is acidity or alkalinity. Usuallu light soils are rich in phosphorus and low in potash and heavy soils are the opposite.