OP
Thanks for your thoughts. I have decided to give this a go! I will base the app around invoicing initially, but wit the added function of timing your work day. I am obsessed with simplicity and functionality so it will be extremely user friendly.I think it varies a lot and depends on the job, but I'd say that a lot of electricians, plumbers and other trades base unplanned jobs at least on an hourly rate. I did the same when I fixed computers as a previous occupation.
Some might have a call out or minimum charge, which could perhaps be set separately and taken into consideration too.
I also vary things slightly depending on whether it's a job for a regular client (landlord with several properties for example), or a new client. Travel is sometimes also taken into account if it's further than normal.
However, regardless of that, a system that kept records of how long was spent at each job would be useful purely for information, even if it was decided later to charge for less time.
There are possibly also tradesmen who work to a fixed price for a particular task, such as installing a kitchen, changing a consumer unit, etc. but those would usually be quoted for in advance anyway.
The basic idea is that you can create a new client, then create a job name for that client, click 'start' and 'end' job, which will auto generate an invoice. That is the basics but I can expand on it if it proved useful.
It would be very helpful to get your feedback as it develops if you don't mind? It may take a month or so to get it all set up but I will keep you posted!