I'd weigh in on the side of holding rent at current levels, or - at most - increasing by a CPI-only inflator. But as with a couple of other posters, I've held my rent at the same level to keep reliable tenants who keep the place well and who wouldn't be able to afford an increase. It does mean I'm charging a bit under the current market rate, but I'd budgetted for that anyway - and tenants who know they're being well treated will (1) stay, and (2) probably treat the place better.
If you have an interest only mortgage which is not being covered by the rent it could be a different story, but if, like a lot of us amateurs, you're making principle as well as interest payments and are still making a profit versus the interest element then I'd leave well alone.