I have found Golden arrow to be good in the past. Although they did keep phoning me at one stage asking me to pay VAT which I already had done.
I know they're not cheap but it sounds more like a typing error to me.
Speak to them in person. Speak to the engineer who worked on it and check if the invoice relates to the time spent.
If it does then you can argue the amount on the grounds of they should have communicated an expected cost to you.
Jim