Welcome to this user forum.
Are you a business customer?
You may be able to receive the wireless signal from the other building if you use a router with an external aerial connector that can be cabled to an outside directional aerial.
None of the BT home hubs have that facility, but you could add a wireless access point that does have an external aerial connector.
The TP- Link TL-WA701ND has a single aerial connector which you could use with the TP-Link TL-ANT2409A 2.4 GHz 9dBi Directional Antenna.
If the buildings are adjacent, you could run an Ethernet cable between them.
Thanks Kieth. Can i confirm - I should buy separately the new router and ariel suggested. If these don't work, I will need to connect this via an ethernet cable?
...and I am a domestic user. What type pf person would fit the ethernet cable? Cheers M
It depends on how permanent you want the cable to be. If it's just a temporary thing, just buy a long cable, and string it up between the buildings. Plug one end into your home router and the other into the wireless hotspot.
For something a bit better, more permanent, and less likely to be tripped over, try any electrician who knows about data cabling. Ethernet cables can be up to 100m long.