Do remember that capital gains tax will take a good chunk of any eventual profit, and if Labour get in I dare say they will increase that on any 2nd or further properties.
We have one 2 bed rental property which has been a moderately good investment so far but we have an extremely good lead tenant who's been there 5 years plus. We always sort out any maintenance issues ASAP, and despite having spent a lot on renovating the flat to a standard we'd have been very happy to live in ourselves, there are still issues - some quite expensive - now and then. We do have a letting agent who will see to maintenance, but they can be a mite slow to act and often charge more, so we've sorted out several things ourselves.
Should add that before buying I did a LOT of research on prices and rental values, and made myself very familiar with the area by walking around at different times/weekdays and weekends. Not that I'm planning on any more, but I'd never buy without doing this. Also, I would never buy a newbuild flat - prices are often so much higher per sq meter than older ones, and ground rent/service charges are frequently very high/extortionate.
IMO a lot of the problems people subsequently have with rental,properties come from a) doing little or no research, b) buying in unfamiliar areas and believing what they are told by estate agents about demand/rental values, and c) over stretching themselves on any mortgage, so that any difficulties with tenants not paying or a void for a couple of months, causes a lot of financial difficulty.
I see from the areas I keep a close eye on - part of SW London and the parts of Oxford where dds live - that a number of landlords are selling up - you can often tell which are rentals - so it would seem that quite a few are feeling that what with the recent tax changes and prices beginning to drop in some areas, it's time to get out while the going's still good.
However, IMO there will always be a market in many areas for good, well-maintained properties, at reasonable rents. Setting a reasonable rent in the first place is IMO important - during my online nosing I see many which are priced on the very greedy side - and then sit empty for a few months while the rent is reduced in reluctant little nibbles until it's reasonable.
An alternative investment would be stocks and shares ISAs - if you both put in the max yearly they can provide quite a good tax free income to either save or re-invest. Best to seek advice on both anyway.