How many visitors for 45kbytes /sec?

I would say that considering the visitors will only spend a small portion of their visit loading pages, probably 10-20 visitors can be visited at a time.

I would not recommend hosting it in your office, though, because there are other risks that could lead to downtime, such as power & DSL outages. You can find hosting for only a few $$ a month, and it will be hosted in a professional and high-tech data center.

Well, quick and rough calculations state with your images being around 200kb, other stuff, text etc, it would likely take one visitor around 5 seconds for the first page hit, then perhaps 4 secs after that.

If you had 10 visitors all at one time, which isn't a lot for a popular website, then it would work ok, and not particularly slow but not particularly speedy either.
And for future proofing, if you ever add something to the site that requires a bit more power, you might run into trouble.
You will also run into trouble of course if the site gets 50-60 hits at one time. It will just bottleneck and grind to a halt and people wont come back.
50-60 hits (and more) can happen easily if you put out an ad campaign, or even if it gets popular.

Also, I would read carefully your ISP TOS, as they can be funny about hosting your own site/server through their lines.
It looks like you are with Demon Internet, and if so they will rent their ADSL from BT, so you should also have to look at BT's TOS for that as well, although Demon's should cover it, it's worth checking rather than getting your internet cut off.

An option, if it's in your office's area, is Virgin Media have 50MB broadband which has an upload speed of 1.5MB (that's mega BYTES, not bits, so its NOT 150kb/s, it's 1,500kb/s)
It's also fibre optic so it's a little better than the ADSL and is usually closer to the max speeds than ADSL claim.

All that said, it can be very involving and time consuming to run it yourself. It takes a bit of setting up, but also will have to be managed. It might run smoothly 90% of the time, but it will require some management even if running ok, monitoring it to make sure things aren't getting bottlenecked, and perhaps spending time tweaking caching options based on the results of the changes you make.

I'm not wanting to put you off, if it's "your kind of thing" then it can be interesting and a good learning curve, but if all you simply want is to save money then it's probably not worth doing. For a server for what you need, you would be looking at around £5 per month, that's about 17 pence a day.

The cost of £5 a month might be worth while compared to the time you will likely spend with doing it yourself, especially if it's only money saving you are thinking of.

EDIT: As you said $70 a month I thought I would give you at least one link

Link to 1&1 Packages

Web Space 1.5GB
Monthly Traffic included 20GB
Connectivety 40GBit
£4.99 per month (I think plus vat)

I'm not particularly suggesting using 1&1, it's just a price example. Their admin area takes a little getting used to, but they are fine as hosting goes.

Is there some particular reason why hosting with those specifics on is dearer?

Well, I'm not sure exactly why you require Tomcat specifically.
Perhaps you do, but if all you want is to run Java on your website, you do NOT need a Tomcat server.

Any basic server that runs on Apache (etc) will be fine.

Instead of guessing, or calculating, why not just try to figure out close to actual numbers, by using a benchmarking utility like 'ab' (Apache Bench) or 'siege'.

They're little command line programs you can tell to hit a specific web address (including post/get data, etc) with a specified number of simulated user and transactions. Tweak the user and transaction input until you're happy with the results, and then you know how many users your application can reasonably handle with the facilities you have in you have in place.

Use the utilities on a different connection and machine with plenty of bandwidth available, so you know it isn't the limiting factor.

Also: You may be able to squeeze a bit more down the line if you've plenty of hardware - by setting up your application server to negotiate http-gzip, compressing content before sending it to the client. Tomcat example setup here: http://www.tek-tips.com/viewthread.cfm?qid=1221613&page=1

Cheers!
Shaggy