@belieb Short answer: letting the property is unlikely to be a good idea.
Long answer:
I assume you bought the other property where you are living now? So you paid the additional 3% stamp duty, right? If you sell the old one within 3 years of buying the new one (and if you don't have any other properties), HMRC will refund you the extra 3%. This alone is a bit of a biggie!
Do you own this new property outright or with a mortgage? If with a mortgage, what rate are you paying?
Where is this old property and how much is it worth? Gross rental yield tend to be ca. 3-3.5% around London, higher up north. A gross rental yield of 5% means that a property worth £100,000 is let for £5,000 per annum. However, the net return can easily be half of that, after you take into account estate agency fees, void periods, taxes (what is your marginal tax rate?), ordinary and extraordinary maintenance expenses (eg this year I have to spend some £15k to replace a rotten French door + to replace the roof).
Quite a few landlords are getting net yields of around 1%, and quite a few are actually losing money because of high maintenance expenses.
If you're considering letting, I assume you don't need the money right away, but would be happy keeping it invested for longer periods, right? If so, if you have a horizon of at least 5-6 years, I'd say that investing it in funds and shares is likely to be a better idea.
There is of course an element of risk, but so there is with letting.
Over a longer period, funds and shares are likely to outperform the return from letting.
Importantly, they are a passive investment - you won't need to chase tenants for payments or to organise the repair of the roof.
Tax-wise, depending on your specific situation, it's likely that the return from funds and shares will be taxed less. The capita gains tax allowance is currently £12,300, meaning you only pay capital gains tax on amounts above that threshold, and the tax rate after that threshold is typically lower than the marginal tax rate at which rental income gets taxed.
Not to mention you can invest in a stock and shares ISA and in a pension (depending on whether you are already maxing these out).