Does this seem a fair cost for such a service or would we be better off buying extinguishers ourselves and just paying for the annual servicing? Is the annual inspections a legal requirement, a recomendation or is it down to risk assessment?
In all honesty you should have got at least 3 quotes if not more even if they are just telephone quotes. You will almost definately save money by buying and having them maintained annually by a competent service technician.
Is the price you are paying all in or do you pay for recharges and replacements?
It is a legal requirement to have your fire extinguishers maintained at least annually.
I would suggest never buy from the internet or catalogues unless you have a competent person to commission and maintain your fire extinguishers.
A good reliable extinguisher company would if you gave them the figures charge more than an catalogue or internet company but they would /should do a far better job than just dumping boxes on your doorstep for you to try and work out what goes where, am I doing it properly, do I know what I am doing, was the extinguisher damaged in transit never to work again, delivered and placed on it's bracket empty, water extinguisher in the mainframe room scenario.
Always buy Kitemarked extinguishers. Some in my opinion are better than others. Trouble is that you won't know what is what.
If someone not competent or incompetent commissions or maintains your extinguishers any saving you made would be a big time loss if and when a fire extinguisher doesn't work when you need it.
Have you had a Fire Risk Assessment? Does it confirm that you need this many extinguishers?
Also, is there a requirement to replace extingushers (and fire blankets) every ten years?
No but if you are renting does it matter? An extinguisher should last at least 5 years but could easily reach 20 years.
You should get an estimate for 5 years to give you an idea.
AnthonyB's suggestion that you could get a new extinguisher every two or three years for £25 (these will be internet prices for a box to be dumped on your doorstep) is in my opinion not really an option for the reasons I have mentioned above.
Unless you or someone within your organisation becomes competent to commission extinguishers my advice would be to always, always, always employ a competent service technician.