We had our chimney swept once and I watched the bloke, since then I do it myself.
Mind you a lot of companies overcharge quite shamelessly no only for the sweeping but also the price/install, when we bought ours we paid just under £800 for the stove and £400 for the fitting whereas when I went into a 'stoveshop' they wanted £2,500 for the stove and install. I also had trouble finding someone to fit it but after ringing around got someone to do it and it turned out to be the son of the bloke whose Brass Foundry I'd worked at, proper HETAS registered and did a fantastic job, Chimney passed the 'leak test' so no need for a liner and the guy even fitted a proper 'pot' and witches hat cowl (not just a wire mesh anti bird cover)
Likewise I wanted a 'beam' for above the fire and was quoted £200 for a bit of wood so went off to a timberyard and got the piece of Oak I wanted for £40 (It isn't functional merely decorative)
Stove is a Franco-Belge Montfort Mk 2 with the modified vent thingy fitted so it is a DEFRA approved exempt appliance but is no longer 'Multifuel' it is Wood Only with that fitted.