Taxi firm information is provided by Letchworth Garden City Tourist Information Office.
A list of firms covering Letchworth, Hitchin & Royston is available here
You may find it helpful to print this off for yourself, especially if you need transport to and from Wallington. There are no buses that go through the village. The taxis will collect you from or take you to Baldock station which is nearer than Letchworth if required.
The taxis are metered (the rates set by the local council) unless you agree a fixed price. We have been informed by one taxi firm that they would expect the price to Wallington from Letchworth to be around £12 or so, and from Baldock for around £8.00 or so. These are not definite prices, and you could check with any of the firms first if you wish.
The major taxi ranks are outside the stations in Letchworth and Baldock.