Well, technically you shouldn't do it yourself because you can change signal levels, to the detriment of your service, by altering any cable lengths. If a tech is doing the alterations, then he can use various methods to compensate for this while he is on-site.
If you don't mess up the signal levels, then you can still introduce noise onto the system if you use cheap cable or poorly made-up connectors.
If either of the above occurs, then you would need a tech visit to rectify it, and bang, you get the £99 charge anyway.
However, seeing as it's only a few inches you need, then you will *probably* be ok if you buy something like this:
http://www.maplin.co.uk/Module.aspx?ModuleNo=29537
Using a pre-made cable with moulded f-plug on one end and f-socket on the other reduces the risk of causing interference that can occur if you use screw-on f-plugs and don't do the job very well.