Hiya,
There are several issues here (I am a pro by the way). Can't be too detailed in my response because there is more to it than the info we currently have. I know this goes beyond the scope of your question, but it opens up other issues.
If my memory is correct (don't have access to my regs book at the mo), any water storage heater with less than 15 litre capacity could be fed from a ring circuit within the regulations, any higher capacity must have independant circuits. A 2.4 or 3Kw instantaneous undersink heater could be fed from a ring. However, you could not really have more than one on each ring circuit, and to be honest it's by far the best practice for them to be on dedicated circuits. I can clarify this if needs be once I get my regs book back.
If there are shared bathrooms, why not have two immersion tanks - one to feed the shared bathroom, one to feed the bedsit sinks.
I'd have to recommend that the issue of electrical capacity is looked at in more depth - a sparks needs to do a site survey. Eight bedsits from a 100A single phase supply seems a bit optimistic to me. Let's think of the numbers - eight undersink heaters, at least one communal immersion heater (?), eight kettles, maybe eight toasters, washing mashine(s), maybe electric showers etc etc. Are tenants going to have electric cookers or hotplates? Much more load.
If the tenants are sharing one communal kitchen and a couple of bathrooms, it may just cope on a 100A supply, I certainly wouldn't commit to it without a site survey, nor based on the info we currently have.
Another issue is whether there are going to be independant electrics for each bedsit - Ie will each one have it's own consumer unit/wiring and it's own private (or supply authority) meter? Also does each bedsit have a kitchinette?
I suggest you get the landlord to get a full survey done, I would think that you'd need at least a two phase, or probably a three phase supply into the building.
This is my first thoughts based on the info given. Pass the buck and ask the landlord to clarify the electrical arrangements. Sorry to be vague but there are a lot of variables that could affect this.
Cheers
paul