So if all the conditions were signed off 18 months ago then you should be well on your way by now. Our in house architects prefer to work with a structural engineer they trust so we let them arrange that but I would have said you should have had completed building regs and construction drawings within 6 weeks of gaining your permission if things were set in motion as soon as the PD had been approved and then you would have been able to start as soon as the conditions were signed off.So, you have satisfied the bat survey and materials, so your planning is all sorted. If that was done in 2017, what's gone on in the last 18 months?!?
Local guy sounds the problem. Dump him. Get proper project manager who will get it priced and done for you.
Is the drainage survey to check the routes of the existing drains, or to test if the soil is suitable for a soak away? Do you know where your surface water and dirty water will go?
I don't expect you to say how much you have spent so far but if I would be concerned if you have spent over £15k from the start of planning to starting work. That would cover planning, architects, bat and drainage survey, structural engineer and building control fees. Unless this is a massive project that I hadn't realised.