Hi Tim,
It depends on what you are doing? If you're fitting wardrobes or painting and decorating you can probably do this yourself. If you want a top quality finish you may want to consider a professional. I guess it depends on the type of tenants you want; if you're looking at the student market the quality of finish may not be that important, but if the tenants are going to by London City professionals.
You must remember as a Landlord you have a number of legal responsibilities which mean that you will need to get someone in. For example, you have duties under the Gas Safety (Installation and Use) Regulations 1998 to arrange maintenance by a Gas Safe Registered engineer for all pipe work, appliances and flues, which you own and have provided for your tenants use.
You must also arrange for an annual gas safety check to be carried out every 12 months by a Gas Safe Registered engineer. You must keep a record of the safety check for 2 years and issue a copy to each existing tenant within 28 days of the check being completed and issue a copy to any new tenants before they move in. For this particular issue you may want to take a look at
http://www.gassaferegister.co.uk/default.aspx?popupcheck=trueFor other services like plumbing and electrics, I think it’s best to get a professional to do the work. The key here is getting someone you have built a longer term relationship with and who is reliable, trustworthy, and not going to charge the earth for simple jobs...!