Thanks for your help everyone.
Al

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Summarising for Google if people are looking later:

Leylandii tree quite near house (3-4 feet).
11 foot tall, 8" wide trunk.
Unlikely to cause subsidence or heave.
Small root ball.
Grows too bloody quickly!

Advice:
- chop it down
- leave 4 foot stump to use as lever to get root ball out