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If you dont wanna get a engineer in, take some out the floor board and see if there is room to move the pipe up and down or else you'll have to get a man in if you cannot do it yourself.
Be careful on that option tho that you dont nick the pipe if doing it with the board in.
Personally id probs lift the board id it was me instead of trying to do it with the board in place, unless you've got boards at the house I was at last week that were a fuckin mission to lift as they had used giant nails and had it under the skirting but yours is long ways so should be ok.
Replacing a single radiator with a double - is there any way I can solve this, other than getting a plumber in to put a step in the pipe?