The typical night-latch that you describe costs less than £10 and doesn't need a locksmith to change it - they are simply fixed onto the back of the door with 4 screws and anyone with a screwdriver can change them.
Thanks guys.
Need to change them so someone who has keys can't get in, so may get away with just a barrel if we can find one that fits - not sure how easy that'd be ?
Will have a go ourselves too - quotes I've had are around £140 + VAT. Worst that can happen is we can't so we just call a locksmith then. Sounds like a plan to me ! LOL