Servicing an under-chassis diesel heater

Question

So my van (Mercedes Sprinter XLWB) has a diesel heater inside a metal box hanging under the chassis by 4x 17mm bolts.

It is connected to air ducting to deliver the hot air into the back of the van.

It is also connected to a fuel line, and electric cables for the thermostat and controls inside.

Finally, it also has a length of exhaust and air intake piping.

The question:

The diesel heater needs servicing (or replacing). This will likely involve taking it to a specialist heater place for testing, or purchasing a similar model. They asked if I could please bring them just the heater, not the whole van.

One side of the metal box is a plate bolted on, but I can't open it with it in-place. The small bolts are accessible, but 2 of them have no room to pull out. So I can't take the side of the box off, to see inside.

I need to detach the entire metal box heater from the underside in a way that is:

1. Safe, in terms of detached cables, pipes, etc
2. Replaceable - ie, I like the current setup, and want to just return the serviced/new heater into the metal box
3. Allows me to continue driving, because I can't be sure it will all happen in a single day.

I am an amateur. I have tools, but no workshop.

What is my strategy?

Answer 1

Posting update in 4 parts, with photos, as an answer, in case it helps anyone

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1. Got seized bolt off with a hacksaw. Tight space, awkward angle, made it seem better choice than a power tool. Other 3 bolts came off with spanner just fine.

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2. The metal box came away just fine. Only electric cable and fuel line was a problem.

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3. Disconnecting the fuel line, and cable-tying it securely up underneath the chassis.

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4. The electric cable did plug into the heater, but the plug itself didn't fit through the hole in the metal box, so some electric cable had to be cut. Hopefully will not repeat that when I come to repair or replace.