Hello. If you've all signed one tenancy agreement, it is likely that you are jointly and severally liable under it. This means that you could be liable for a breach caused by one of the other tenants. The risk you run is that this couple refuse to leave when the tenancy ends. The landlord might then have to take eviction proceedings, and he may be entitled to sue you and the others for his costs or losses because you're all liable for the breach that caused the problem. Other issues could arise, such as non-payment of their share of rent and bills etc. Personally, I would not allow the subletting to proceed without consent.