The wayleave agreed when it was installed would normally give access rights to BT staff.
BT maintain a register of all wayleaves. To check on exactly what the wayleave in respect of 'your' telegraph pole states, phone 0800 671408 or email: [email protected]
my inlaws had a similar problem but with electricity poles. the poles and the lines belong to bt but they require your permision to enter your land to inspect them unless they already have it. you may also b entitle to a small amount of rent from them unless a previous occupier has already made an agreement with them for access and a fixed term payment.