Are you going to DIY this? How big’s your inverter, kW-wise?

@editor yes I need to nobody is interested in fitting a few panels and certainly not on the garage. Too much bother.  I posted a photo of the data tag on the inverter, not sure what figures you need but growatt told me plenty of capacity.  I think I’m only allowed to go to 4kw before I need to inform the energy supplier.

@david999 I’ll be super interested to see the progress.

I think @transparent might be able to offer some helpful insights.

To be frank,

If you don't know how to wire and are relying on a forum to talk you through it, you really shouldn't be doing it. DC volts will easily kill you! Isolating in the wrong order can break things. Get an electrician in. Mount the panels run suitable cables if you must, but leave the professional to do the wiring connections.

Use ballast trays to hold the panels, so no drilling required in to roof.

@david999

1: That inverter is a Growatt SPH3600TL. Its output is already limited to 3.68kW (16A) and no further consent is required from your DNO to add more panels.

2: Each of the two solar inputs is rated 120-550v DC at 13.5A max.
That's vastly more than the possible output from the inverter.
The inverter sorts this out automatically.

3: You already have 2.6kW (max) PV panels on one of the two inputs.
You could add a similar quantity to the other input and enjoy less of a tail-off from generation on overcast days.

4: Solar panels themselves are pretty cheap.
Look at these Longi panels, available from Midsummer.
The main costs are the scaffolding, labour and support structures required for roof mounting.
I can address the subject of aluminium roof rails later if you require that knowledge.

5: It's unlikely that you'd require anything of greater current-capability than the common 4mm (double insulated) solar cable.
You can buy this and the required MC4 connectors from Midsummer here, or a general electrical supplier like TLC-Direct here.
You need some tools, which I'll address separately if you decide to go the DIY route.

6: It's possible that the black/grey rotary isolator switch to the left of that inverter is already a 4-pole unit.
So a second pair of 4mm wires can be connected to it.
Personally I'd prefer to see more than just that switch.
I have fuses, over-current trips and lightning suppression on each solar array!

Let me stop there and hear what you think.

The comments from @johnmo about safety are well made.
I've deliberately omitted comments on safety aspects at this stage, but I have hands-on experience of how to do this work safely.

@johnmo I’m an engineer but no experience in solar. I wouldn’t install a complete system but adding another string shouldn’t be that challenging.

@editor I will keep you informed but busy finishing off my house still after the flood.  The heating got sorted and the samsung bill refunded but they were at the end of their tether with me but I reminded them that I was the victim.   But altogether the room full of witchcraft became something I understand now thanks to you guys

Another source of parts is for PV is City Plumbing, seem very good price wise.

@transparent

@johnmo they seem to be fairly cheap, the ballast boxes are as much if not more.  Being engineer minded and able to weld a frame together I’m thinking much cheaper, if anybody has done this before I would appreciate images of how they went about it. I would be happy with another 4 panels.

Not worth the effort to make your own 4 panel system is only £315. Or you just copy to make your own angle.

https://voltaconsolar.com/products/solar-panel-mounting-flat-roof-ballast?variant=47416621564238