Some good points there

I became a father myself 14 months ago now, after 10 years of running Omega's now that there is a MrsT and Little MissT, DIY service does become less attractive. (mainly time being the issue) Given the situation I'm now in, both my Omega's combined in value would barely crack £1k, so really sod all point in selling them. All adverts I see for new cars, easily start at £199/m - Hence my post on the economics of it all.
As you say, even a brand new car can get issues, and easily get stung by £300 bill. Something thanks to OOF, Omega issues could be sorted for a fraction of that.
I would not consider the LPG a heavily modified system personally, it still drives just the same, the filler point is just different. But I see your point, I get many odd looks when filling up with LPG.
ISOFix, well that's an interesting one too. Neither of mine have it, but the Omega is a big and fairly safe car. I've been quite happy with the seat belt method using a Maxi-Cosi Easybase2, it's very secure. The law around child seats is a bit of a mine field, there is talk if rear facing becoming law for much longer. Also, ISOFix vs the so called i-Size which is being introduced:
http://www.huffingtonpost.co.uk/2015/04/04/child-car-seat-confusion-our-guide-to-the-new-regulations-and-products_n_7331866.htmlGiven the miles each car does (~3k for 3.2 - ~7k for 2.2 and ~5k on the bike), for me new cars do not make sense. If forced, but hoping not for at least 3 years, will most likey go down the route of a Merc CLS 320 for me, around 2006-9 Vintage and a Merc A or B Class for MrsT.
This appeals to me:
http://www.autotrader.co.uk/classified/advert/201507054933714?radius=1500&channel=cars&make=mercedes-benz&search-target=usedcars&maximum-mileage=up_to_80000_miles&fuel-type=diesel&onesearchad=used%2Cnearlynew%2Cnew&sort=default&page=5&price-from=6000&postcode=tw89de&model=cls&logcode=pand this for MrsT:
http://www.autotrader.co.uk/classified/advert/201507185307210?channel=cars&onesearchad=used%2Cnearlynew%2Cnew&maximum-mileage=up_to_80000_miles&fuel-type=diesel&search-target=usedcars&make=mercedes-benz&sort=default&radius=1500&model=a_class&postcode=tw89de&price-from=6000&page=1&logcode=pBoth those cars new for me to buy would be significantly more, CLS was what, £50k new, the A, around £15?
So for £15k combined, got £60k's+ worth of car new.
Downside of course is need to find an independent to service them, plus fact things could go wrong. (but my miles are fairly low) - But I think I'd much rather have a CLS, enjoy driving that and factor in those costs, than drive a 1.4 Astra drab box