Unfortunately its not cost effective to service the units at your premise, however there are service centres across the country which you can either post or drop the unit off and have it serviced, if you let me have your postcode I can advise you of the contact details for your nearest Bissell Service Centre.
Most steam cleaners are sealed units so there are limits on what can be done, by in large the service centres will just put descaller through it to remove lime scale.
With regards ***** ***** potentially blocked attachment, I would soak it it boiling hot soapy water, then see if the attachment starts working, if that fails you could try leaving it soaking in drain cleaner, to break up what is likely to be lime scale.
In summary you could take it into a service centre who would charge around £30 to look at it, but there is very little they can do that you cant due to it being a sealed unit.
Please let me know if you would like details for your nearest service centre, or need anything clarified.