Intermediate joists support

We are looking to remove a ground floor wall which appears to provide intermediate support for the first floor joists. From looking at various building regs tables it appears that removing the wall may cause the span to exceed the maximum allowed amount.

The house wasn't exactly built to the greatest standards (due to the era it was built). The joist spacing is a bit inconsistent where it passes over the top of the wall, but seems to average 400mm between joist centres. The overall span without the intermediate wall would be around 4.875m, which is probably either right on or just over the limit, depending on the timber grade I suspect. The first floor walls are built from concrete blocks, seemingly on top of floorboards only.

If intermediate support is needed, which I think it will but would appreciate any input regarding that, then we can put a RSJ in place? I would think that one of the lightest RSJs would be sufficient, but also need advice on that? As this is the only new opening we are creating, and it is not a particularly big wall, nor is there a wall directly above, I would prefer to avoid using a structural engineer if possible.

I've included a scaled diagram showing dimensions - hopefully it is clear enough.

What would the span of the new beam be?

The intermediate support beam, if required, would be approx 2.5m long.

You would be looking at something like a 178 x 102 UB.
This is on the basis that it woud be supporting ½ the floor load, plus the majority of the blockwork wall above.
You would also be advised to look in the roof space, to check that the blockwork wall itself is not supporting anything of the roof
structure (sometimes there is a prop off this wall supporting a roof beam).
BTW, as this would be a structural alteration, you would strictly need to get Building Control involved.