Complex sentence: Buy-to-let mortgages have been on offer in the UK since the late nineties; they are specifically designed for investors to borrow money to purchase property in the private rented sector in order to let it out to tenants.
Split into simpler sentences: Buy-to-let mortgages have been on offer in the UK since the late 1990s. They are specifically designed for investors to borrow money to purchase property in the private rented sector in order to let it out to tenants.