It is tempting to say your boss is a tosser, but that is not constructive nor helpful.
Since you have worked freelance for them before, my guess is that he should know what you are "worth", and now he might be trying to get away with less.
I would suggest you stand your ground, though, and see this constructively. Politely ask and try to figure out why he thinks you're not "worth" 18000 quid, as was his suggestion. Ask what you would have to do to convince him otherwise.
There is a possibillity that someone higher up are looking at money in, money out, and that your boss is only the messenger?
Don't use the argument of paying for sparks, broadband etc. I don't think this is what it is really about.
There is a possibillity that they expect you, as an employee, to behave slightly differently than as a freelance:
I hate to say this, but communication might help - you know, status mails, sketches, cc anyone involved, ask questions. Don't pester people though, but give them a little more info than they technically need, about where things are at, and where they're heading. Be quick at answering their mails. Call them.
When you pay a freelance piece-by-piece, you don't care how or how long it takes, really. When you employ someone though, people seem to like to feel to be in "control" of them.
Keep reasonable track and timesheets for the jobs you do, and send them in now and then.
I have worked both in Northern Ireland and the Republic, and I found that the communication expected of me was different than what I am used to. My employers there seemed not to have the same trust and faith as I am used to, and the was little freedom to manouver.
I was also, in one job, told they had to deduct my pay. I had kept meticolous lists of what I had done at any given time. He was not interested in seeing this. I politely refused ("deduct my pay? don't think I'd be very interested in that"), the guy said I should "think about it, and we'll have a meeting again wednesday". He didn't show up. He didn't show up the next day either. Nor the day after that. He didn't call, he didn't leave a message. I honestly thought f*** this, packed my tools and left, essentialy, the country. Not a solution for you though :-)
Wish you all the best. It seems indeed you make quality stuff. And keep in mind that it is probably more stress for them to replace you, with a new person where they have to go through that whole getting-to-know, checking results-hassle.
You seemed to be skilled, they know you, and that counts for something.