You don't state what seal you mean as there are two, one at each end of the engine.
The front seal is of the lip type (spring loaded neoprene), and is held in place by the sump, (half), and front timing cover, (other half). However, if you look on the engine diagram, you will see that the actual seal bears on a separate spacer, which is pushed on to the crank shaft and has its own O-ring seal to the crankshaft. So you take off the crank pulley, damper, damper cone, and sump, pull off the spacer, then the seal should drop down to permit its removal. However, I have only replaced a seal when the engine was dismantled, so maybe Doug has done one and will post. Of course this method may not work and you will need to remove the timing cover. This is possible without removing the head, but the sump has to come off first and the studs fastening the cover to the head have to come out. See if you find a decent engine layout diagram on the internet - I found one at www.xks.com
The rear seal is an asbestos rope-type that needs the engine to come out and the crankshaft to be removed, a BIG job !! There is a conversion kit for a modern lip-type seal, but this needs the crank to be ground as well to make a smooth contact surface for the seal.
SNG Barratt - UK | Accessories | Crankshaft Rear Oil Seal Conversion
The two halves of the asbestos rope seal are very cheap, it is the labour that makes the job expensive. There is also a definite knack in fitting one, and you really need to use the special sizing tool, although I didn't and then suffered from leakage ever after.