HI R2,
I do indeed run a LTD co. Have done for 12 years now. I am ok with IR35 as I do have more than 1 customer. I have my day job (5 days a week) through the agancy plus my company is set up as a supplier to the end company and I have 2 other small contracts directly with them. I do 4 days resource for one customer and 2 days for another for which I invoice seperately.
I also do some freelance web work for one or two people locally. I does'nt pay much but helps as I can demonstrate multiple customers.
I am still in an ongoing 7 year battle with the Inland Rev which is soon to be resolved but have an ace accountant (who used to work for the IR)..
Ah well, you're fully set up and savvy, you should have no problem. I know a number of contractors who treat a contract like a job, work the five day week, don't even have a business phone at home or accounting software to make their lives easy, particularly for VAT returns, etc. As they are not really running a business they are at great risk from IR35. Even so, most of them seem to get away with it.
Fortunately I've never had a contract longer than around two years (I'm usually doing European implementation projects, when the implementation's done, I'm done!) so I tend to move on before I feel part of the furniture! Hope you get the IR sorted out, there is nothing worse. I used to wake up in the middle of the night absolutely furious that, contrary to the principles of British justice, I had to prove myself innocent when it was absolutely clear I had no case to answer. My lawyer made sure that everything they asked was queried, aparently the IR cost these inquiries up, so much for a letter, etc, and review at the end whether it has been worth it. The more you cost them, the less likely they are to be back. They take their own sweet time doing these things which puts a tremendous strain on the poor individual undergoing the inquisition, I'm sure it's done on purpose.
I've spread things as far as I can, apart from a support contract with a major company which I do with a partner and a separate company (direct contract, no agencies involved!), I do a bit of web hosting on a leased server, doesn't make any money but offsets costs and hey, it's another half a dozen customers, have become a Quickbooks Professional advisor and sell EDI software (which we interface with Quickbooks, using C# and .NET courtesy of my son), programming and EDI for numerous customers on an ad hoc basis, and various other bits and pieces. Anything to earn a crust

. I do tend to work 7 days a week quite a lot though. Fortunately I enjoy the work and most of it I can do from home.
Good luck, I'm sure you'll have no trouble and there's nothing more fun than getting into a new contract and hitting the ground running!