150 hours is generally considered to be about the break even figure for sole ownership vs renting.
This can be readily illustrated by taking Genghis's excellent (as usual) advice to consult Afors. A current Afors advert for a Cessna 150 based at Leicester puts the total monthlies (which would include hangarage, insurance, Annual etc) at £320/month and the wet costs (which would include fuel, engine replacement, 50 hr maintenance etc) at £80/hour.
If it was your aircraft and assuming costs the same, total for 150hrs/year would be (£320x12) + (£80x150) = £15,840, equivalent to about £105/hour.
If you were to fly it for only 100 hours/year however, costs would be (£320x12) + (£80 x 100) = £11,840, equivalent to about £118/hour.
Neither hourly costs are significantly different to the £112 you were quoted for renting from Stapleford.
So as I said above, 150 hours/year means the cost of ownership will be about the same as the cost of renting, so you have to consider pros and cons of each.
Benefits of owning your own over renting would be unlimited availability.
Benefit of renting over ownership is no capital tied up, no unexpected bills (e.g. Annual costs bigger than budgeted for),someone else responsible for arranging maintenance, insurance etc, and no aircraft to sell if you decide to fly something else/stop flying etc.
Of course there are far cheaper ways of hour building, and far more interesting types available to fly - have you considered sole ownership or a share in something on an LAA permit? You could likely half the above costs on something that would fly faster and further than a PA38/C152. For example my Alpi Pioneer will cruise at 130 Kts and 150hrs/year costs me in total around £8K pa for 150 hours including all fixed and flying costs - so around £50/hr all in.