For a small window like that I'd budget half a day's labour plus materials. £400-500 all in is about right depending on your area. The lintel swap is the bit that takes the time, not the brickwork itself.
If the existing lintel is rotten, definitely replace it rather than just bricking over the top. It'll only cause problems down the line if you leave it.
Too far to travel for me, the 3/400 quotes are about right for a short days work, but i would not need a lintel to brick up a window, as the above post (from a builder) suggested, the brick kinda support themselves
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