I really feel for you - my wife grumbles about the amount of space the B takes up in our small barn, and how there's never any room for her to put anything else in there, but I feign selective deafness on such occasions. I had mine off the road for at least 3 years when it needed mechanical attention and I didn't have the time to sort it out. That lay-up doesn't seem have done it any harm, but it was undercover and dry at all times. I wouldn't have wanted to keep it outside, even under a cover, as the damp kills them. I'd speak sweetly to your mum about using her garage. It would be a great shame to part with it. If it's not in tip-top condition you won't get much for it, and if you spend money on it in order to sell it, you risk not recouping the cost. And if you ever wanted to run another one, would you be able to find one as good as the one you have?
If you can sort out the dry storage issue, it won't cost you anything to lay it up, and it's not costing you anything in depreciation. If you can afford to buy a replacement without selling the B, that's what I'd do, and then you have the option of running it again as & when the opportunity arises. I tax mine for 6 months in the summer, and my insurance costs me less than £100 pa, so as playthings go, it's very cheap. The trade-off is that my daily drive is a little Panda 4x4 that also costs relatively little to fuel, tax & insure. I'd probably have a much harder job justifying the B to 'er indoors if I ran anything less practical/more costly as a second car (she just about tolerated the DS3 as it was cheap to run and surprisingly capacious). Having said that, she knows how much pleasure I get from owning the B ("Sometimes I think you care more about that bl**dy car than me") and I reckon she thinks that, as vices go, it's a relatively harmless one. My ace-in-the-hole argument is that Bs are becoming rarer, so keeping it is actually a good investment, but secretly I'm not entirely persuaded by that argument as they seem to sell for buttons unless in tip-top condition, which mine isn't.