Did they guarantee a result by a particular time?
OK. But was there a clear time limit agreed?
Sorry for the delay.
If this was agreed in March then I do think you have waited a reasonable time.
Although it would have been helpful if you had agreed a time deadline the default position of the law will be that you only have to wait for a reasonable period of time.
You really need though to make time of the essence in the case. Send them a letter setting down that time is now of the essence in this case. You expect the work to be done within 28 days of the date of the letter and in default you will sue to recover your deposit and seek costs and interest.
If they do not then you can sue at the small claims court.
Can I clarify anything for you?
Oh yes, mostly people do.
the small claims court is designed for the lay person.
I would send them a fresh letter before action.
You can only claim your actual fees which would be about £50.