Staircase

I have an old Victorian house and the spindles are not the original ones. I would like to replace them, but unfortunately the base and hand rail, which I was only going to strip does not have the usual cut out to accomodate a spindle, eg in order to countersink the base and top of the spindles and spacers. Therefore should I use a router to widen the gap or just nail the spindles into place - which I presume would have to be cut very accurately. If this is the case is there a neater way of doing this so that the pins would not show?
Car

you could use a string cap [basicly a bit of wood with a trench top and bottom]with the grooves to match your selected spindles and string and use spacers

and please note a sphere 100mm across [childs head size]must not pass through any gap on the stairs to comply with bulding regulations

Not quite sure what you mean by a string cap (where does the string come into it). Basically the base rail has a very thin cut out, not wide enough for a spindle, and the underside of the hand rail has no cut out at all. Do you mean to add another piece of wood along the top of the base rail and underside of hand rail, with a wide trench cut into it. Surely I might as well change the base rail in that case, as this piece of wood would have to run along it anyway and also include all the bends etc?

Car

the cap has a slot underneath which locates it on top of the string
say your string is 1 1/4 inch wide you can take a piece of 2 by 1par[20 by 44mm] you cut a trench or groove central 1 1/4 wide full length about 6mm deep full length off the string router a rebate on the top for your spindles

still as clear as mud eeehh