I do agree with Julian to a degree - it is good to have an indepth knowledge about a particular market and area - BUT diversification means you spead the risk. Even what you see now as a no-brainer could with little warning become a liability.
For instance - about 10 years ago as our first venture we brought a couple of run down student houses in manchester for £25k (deposits down of 2k). We got local scallies! to patch them up for a few thousand pounds, put in cheap furniture and rented them out to 4 students, making a HUGE profit. Our mortgage was £200pcm, they paid £750pcm.
A great deal! We saved every penny we had, we viewed hundreds more, but we were guzumped, outbid, had emergency personal bills so ran out of money. Lucky, as it would happen.
As 2 years later major builders started building luxury student accomodation blocks. It seemed mad, students paying £85 a week to live in a bedsit instead of £45pw in our house. But, within 5 years pretty much all students now live in these nice trendy blocks.
Couple that with a few murders in that area of Manchester, HMO legislation which costs a fortune, the requirement for luxurious housing and furniture, lack of cheap labour - these houses are pretty much unrentable except to the DSS where allowable rent is only £300pcm.
As for selling them, luckily we sold a few years ago for a very good profit, but people who didnt have seen these houses rise from £25k to £125k and recently unsellable at £50k.
Anway, my message is dont put all your eggs in one basket. You can never legislate for what might happen even if you become an 'expert' in one particular location/field.