That bolt under the bottom bracket definitely looks like a drain plug (what else can it be?) so temporarily remove it and see if anything comes out when the bike is the right way up. Normally the bolt should be in place to stop the hole being a point of water ingress. The hole will also be useful for squirting in some anti-corrosion treatment.
Going back to your question about tyres http://thorncyclesforum.co.uk/index.php?topic=14592.msg109480#msg109480, if they feel unstable when cornering then they are most likely too soft. You need to get a fairly good pressure gauge. I wouldn't trust one that costs less than £5 or is on a very cheap hand pump but a £20 floor pump with a dial gauge should be fairly accurate. Manufacturers give recommended pressures although the ideal pressure for an individual depends on their weight and preference in the balance between rolling resistance (higher pressure) and comfort (lower pressure). Harder is better until you start feeling every imperfection in the road surfaces.
There's a bewildering range of Schwalbe Marathon tyres with the only thing in common being the name plus greater puncture resistance compared to lighter, more supple tyres. Your options will be reduced by your wheel size. Marathon Plus is in the middle of the range. Marathon Supreme is smoother and should have better rolling resistance while, at the other end of the range, the Marathon Mondial has a chunky tread for extra grip on loose surfaces.
Yeah I’m feeling a lot better about the tyres and cornering
I think you see right that the front was too soft . When the bike was delivered one of the tyres was flat so had to pump it up . It may have been the front I’m not sure .
But yeah , I had no idea what pressure the tyres were after pumping then both with new tubes . I was over cautious and pumped them surprisingly low which I only found out after a got a digital pressure gauge which I’m really impressed with.
A good pump came with bike but it seems like it’s ceased and I can’t turn it round to the schrader side for the new schrader valve tubes I bought , so got a cheapo from Halfords just to get by , but I’m struggling to get them up to 40 psi 😄 , my arms feel like they will explode . Thorn recommended 50 for rear and 45 for front for 1.75” tyres. I don’t even know if that figure is loaded or unloaded 🤔
I bought a pair of schwalbe marathon plus tours in 2.0” as I fancied a little more width and a little more grip for off-road .
I haven’t put them on yet as I wanted to give the 1.75” tyres that are currently on a proper go and see what I think of them . The rear is a marathon plus , and the front is a softer panaracer , it’s a pity they were not the same as I think that would be a better way to test the 1.75” size
I have been going out for long runs and increasing the tyre pressure each time . Not sure if I’m noticing a massive difference. Yet. Maybe once I get up a bit higher /hard I will have a better idea.
When I first put on the new schrader tubes I had pumps it up to 16 psi ! Not realising how low it was even it felt ok. I knew it was on the lower side but would never have thought it was that low. And it ran fine at that pressure. Then I read on schwalbe site that the traditional thumb press test which is normally quite reliable doesn’t work for the marathon tyres