Okay - The way to keep solicitor's fees down is to do as much work / fact finding of your own as you can before you go and see someone.
Get a coloured photocopy of your deeds and the land registry documents to take with you.
Make a bullet point list of the salient points.
When you make an appointment to see a solicitor, make it clear who it is that you will be acting against. Some firms will be conflicted out as they make already represent the developer and you don't want to waste time.
I would write a letter to the Land Registry, or ask the solicitor to do so, asking a simple yes and no question : does that private access belong to your property. Forget the others, unless you want to club together to share legal costs.
Send the letter registered post or Special Delivery and ask for an answer within a fortnight. Do not mention the legal action at this point.
If you get a yes, that the access if yours then you or your solicitor need to send a copy of that to the planning committee and to the developers by Special Delviery and tell them to remove the hoarding immediately or legal action for trespass will follow.
If you get a no, then go back to the solicitors who acted for you when you bought the property and find out why their due diligence on the sale did not bring this matter to light.
The age of your property is irrelevant if you or any previous owner been using the access unchallenged for over 12 years.
When you are talking to the planning committee mention the fact that the developer is already making a nuisance by claiming land that isn't, apparently, theirs. Query why they are doing this.
Also, query whether anyone on the planning committee has a vested interest on seeing the development go ahead. If so, who? What is their interest and why have they not been conflicted out?